diff --git a/apps/miniapp/src/lib/ledger-helpers.ts b/apps/miniapp/src/lib/ledger-helpers.ts
index 892193e..ace415d 100644
--- a/apps/miniapp/src/lib/ledger-helpers.ts
+++ b/apps/miniapp/src/lib/ledger-helpers.ts
@@ -20,18 +20,22 @@ export type UtilityBillDraft = {
   currency: 'USD' | 'GEL'
 }
 
+export type ParticipantShare = {
+  memberId: string
+  included: boolean
+  shareAmountMajor: string
+  sharePercentage: string
+  lastUpdatedAt?: number
+  isAutoCalculated?: boolean
+}
+
 export type PurchaseDraft = {
   description: string
   amountMajor: string
   currency: 'USD' | 'GEL'
   splitMode: 'equal' | 'custom_amounts'
   splitInputMode: 'equal' | 'exact' | 'percentage'
-  participants: {
-    memberId: string
-    included: boolean
-    shareAmountMajor: string
-    sharePercentage: string
-  }[]
+  participants: ParticipantShare[]
 }
 
 export type PaymentDraft = {
@@ -164,6 +168,181 @@ export function paymentDraftForEntry(entry: MiniAppDashboard['ledger'][number]):
   }
 }
 
+/**
+ * Rebalance purchase split with "least updated" logic.
+ * When a participant's share changes, the difference is absorbed by:
+ * 1. Auto-calculated participants first (not manually entered)
+ * 2. Then the manually entered participant with oldest lastUpdatedAt
+ * If adjusted participant would go negative, cascade to next eligible.
+ * Participants at 0 are automatically excluded.
+ */
+export function rebalancePurchaseSplit(
+  draft: PurchaseDraft,
+  changedMemberId: string | null,
+  newAmountMajor: string | null
+): PurchaseDraft {
+  const totalMinor = majorStringToMinor(draft.amountMajor)
+  if (totalMinor <= 0n) return draft
+
+  let participants = draft.participants.map((p) => ({ ...p }))
+
+  if (changedMemberId !== null && newAmountMajor !== null) {
+    const changedIdx = participants.findIndex((p) => p.memberId === changedMemberId)
+    if (changedIdx === -1) return draft
+
+    const newAmountMinor = majorStringToMinor(newAmountMajor)
+
+    if (newAmountMinor > totalMinor) {
+      return draft
+    }
+
+    const oldAmountMinor = majorStringToMinor(participants[changedIdx]!.shareAmountMajor || '0')
+    const delta = oldAmountMinor - newAmountMinor
+
+    participants[changedIdx] = {
+      ...participants[changedIdx]!,
+      shareAmountMajor: newAmountMajor,
+      lastUpdatedAt: Date.now(),
+      isAutoCalculated: false
+    }
+
+    if (delta === 0n) {
+      return recalculatePercentages({ ...draft, participants })
+    }
+
+    const included = participants
+      .map((p, idx) => ({ ...p, idx }))
+      .filter((p) => p.included && p.memberId !== changedMemberId)
+
+    if (included.length === 0) {
+      return recalculatePercentages({ ...draft, participants })
+    }
+
+    let remainingDelta = delta
+    const sorted = [...included].sort((a, b) => {
+      if (a.isAutoCalculated !== b.isAutoCalculated) {
+        return a.isAutoCalculated ? -1 : 1
+      }
+      const aTime = a.lastUpdatedAt ?? 0
+      const bTime = b.lastUpdatedAt ?? 0
+      return aTime - bTime
+    })
+
+    for (const p of sorted) {
+      if (remainingDelta === 0n) break
+
+      const currentMinor = majorStringToMinor(participants[p.idx]!.shareAmountMajor || '0')
+      let newValue = currentMinor - remainingDelta
+
+      if (newValue < 0n) {
+        remainingDelta = -newValue
+        newValue = 0n
+      } else {
+        remainingDelta = 0n
+      }
+
+      participants[p.idx] = {
+        ...participants[p.idx]!,
+        shareAmountMajor: minorToMajorString(newValue),
+        isAutoCalculated: true
+      }
+
+      if (newValue === 0n) {
+        participants[p.idx]!.included = false
+      }
+    }
+  } else {
+    const included = participants.map((p, idx) => ({ ...p, idx })).filter((p) => p.included)
+
+    if (included.length === 0) {
+      return { ...draft, participants }
+    }
+
+    const count = BigInt(included.length)
+    const baseShare = totalMinor / count
+    const remainder = totalMinor % count
+
+    included.forEach((p, i) => {
+      const share = baseShare + (BigInt(i) < remainder ? 1n : 0n)
+      const existing = participants[p.idx]!
+      participants[p.idx] = {
+        ...existing,
+        shareAmountMajor: minorToMajorString(share),
+        ...(existing.lastUpdatedAt !== undefined ? { lastUpdatedAt: existing.lastUpdatedAt } : {}),
+        isAutoCalculated: true
+      }
+    })
+  }
+
+  return recalculatePercentages({ ...draft, participants })
+}
+
+function recalculatePercentages(draft: PurchaseDraft): PurchaseDraft {
+  const totalMinor = majorStringToMinor(draft.amountMajor)
+  if (totalMinor <= 0n) return draft
+
+  const participants = draft.participants.map((p) => {
+    if (!p.included) {
+      return { ...p, sharePercentage: '' }
+    }
+    const shareMinor = majorStringToMinor(p.shareAmountMajor || '0')
+    const percentage = Number((shareMinor * 10000n) / totalMinor) / 100
+    return {
+      ...p,
+      sharePercentage: percentage > 0 ? percentage.toFixed(2) : ''
+    }
+  })
+
+  return { ...draft, participants }
+}
+
+export function calculateRemainingToAllocate(draft: PurchaseDraft): bigint {
+  const totalMinor = majorStringToMinor(draft.amountMajor)
+  const allocated = draft.participants
+    .filter((p) => p.included)
+    .reduce((sum, p) => sum + majorStringToMinor(p.shareAmountMajor || '0'), 0n)
+  return totalMinor - allocated
+}
+
+export type PurchaseDraftValidation = {
+  valid: boolean
+  error?: string
+  remainingMinor: bigint
+}
+
+export function validatePurchaseDraft(draft: PurchaseDraft): PurchaseDraftValidation {
+  if (draft.splitInputMode === 'equal') {
+    return { valid: true, remainingMinor: 0n }
+  }
+
+  const totalMinor = majorStringToMinor(draft.amountMajor)
+  const remaining = calculateRemainingToAllocate(draft)
+
+  const hasInvalidShare = draft.participants.some((p) => {
+    if (!p.included) return false
+    const shareMinor = majorStringToMinor(p.shareAmountMajor || '0')
+    return shareMinor > totalMinor
+  })
+
+  if (hasInvalidShare) {
+    return {
+      valid: false,
+      error: 'Share cannot exceed total amount',
+      remainingMinor: remaining
+    }
+  }
+
+  if (remaining !== 0n) {
+    return {
+      valid: false,
+      error: remaining > 0n ? 'Total shares must equal total amount' : 'Total shares exceed amount',
+      remainingMinor: remaining
+    }
+  }
+
+  return { valid: true, remainingMinor: remaining }
+}
+
 export function defaultCyclePeriod(): string {
   return new Date().toISOString().slice(0, 7)
 }
diff --git a/apps/miniapp/src/routes/ledger.tsx b/apps/miniapp/src/routes/ledger.tsx
index f848e6a..5b65fed 100644
--- a/apps/miniapp/src/routes/ledger.tsx
+++ b/apps/miniapp/src/routes/ledger.tsx
@@ -18,9 +18,13 @@ import {
   purchaseDraftForEntry,
   paymentDraftForEntry,
   computePaymentPrefill,
+  rebalancePurchaseSplit,
+  validatePurchaseDraft,
+  calculateRemainingToAllocate,
   type PurchaseDraft,
   type PaymentDraft
 } from '../lib/ledger-helpers'
+import { minorToMajorString } from '../lib/money'
 import {
   addMiniAppPurchase,
   updateMiniAppPurchase,
@@ -361,6 +365,9 @@ export default function LedgerRoute() {
     draft: PurchaseDraft,
     updateDraft: (fn: (d: PurchaseDraft) => PurchaseDraft) => void
   ) {
+    const remaining = () => calculateRemainingToAllocate(draft)
+    const validation = () => validatePurchaseDraft(draft)
+
     return (
       <div
         class="split-configuration"
@@ -380,7 +387,8 @@ export default function LedgerRoute() {
                     updateDraft((d) => {
                       const newParticipants = [...d.participants]
                       newParticipants[idx()] = { ...participant, included: checked }
-                      return { ...d, participants: newParticipants }
+                      const updated = { ...d, participants: newParticipants }
+                      return rebalancePurchaseSplit(updated, null, null)
                     })
                   }}
                 />
@@ -392,13 +400,15 @@ export default function LedgerRoute() {
                     placeholder="0.00"
                     value={participant.shareAmountMajor}
                     onInput={(e) => {
+                      const value = e.currentTarget.value
                       updateDraft((d) => {
                         const newParticipants = [...d.participants]
                         newParticipants[idx()] = {
                           ...participant,
-                          shareAmountMajor: e.currentTarget.value
+                          shareAmountMajor: value
                         }
-                        return { ...d, participants: newParticipants }
+                        const updated = { ...d, participants: newParticipants }
+                        return rebalancePurchaseSplit(updated, participant.memberId, value)
                       })
                     }}
                   />
@@ -410,24 +420,21 @@ export default function LedgerRoute() {
                     placeholder="%"
                     value={participant.sharePercentage}
                     onInput={(e) => {
+                      const value = e.currentTarget.value
+                      const percentage = parseFloat(value) || 0
+                      const totalMajor = parseFloat(draft.amountMajor) || 0
+                      const exactAmount = (totalMajor * percentage) / 100
+                      const newAmountMajor = exactAmount > 0 ? exactAmount.toFixed(2) : ''
+
                       updateDraft((d) => {
                         const newParticipants = [...d.participants]
                         newParticipants[idx()] = {
                           ...participant,
-                          sharePercentage: e.currentTarget.value
+                          sharePercentage: value,
+                          shareAmountMajor: newAmountMajor
                         }
-
-                        // Calculate exact amount based on percentage
-                        const percentage = parseFloat(e.currentTarget.value) || 0
-                        const totalMajor = parseFloat(d.amountMajor) || 0
-                        const exactAmount = (totalMajor * percentage) / 100
-                        const nextParticipant = newParticipants[idx()]
-                        if (nextParticipant) {
-                          nextParticipant.shareAmountMajor =
-                            exactAmount > 0 ? exactAmount.toFixed(2) : ''
-                        }
-
-                        return { ...d, participants: newParticipants }
+                        const updated = { ...d, participants: newParticipants }
+                        return rebalancePurchaseSplit(updated, participant.memberId, newAmountMajor)
                       })
                     }}
                   />
@@ -436,6 +443,19 @@ export default function LedgerRoute() {
             )
           }}
         </For>
+        <Show when={draft.splitInputMode !== 'equal' && draft.participants.some((p) => p.included)}>
+          <div
+            style={{
+              'font-size': '12px',
+              'margin-top': '4px',
+              color: validation().valid ? '#22c55e' : '#ef4444'
+            }}
+          >
+            {validation().error
+              ? validation().error
+              : `Remaining: ${minorToMajorString(remaining() > 0n ? remaining() : -remaining())} ${draft.currency}`}
+          </div>
+        </Show>
       </div>
     )
   }
@@ -456,7 +476,28 @@ export default function LedgerRoute() {
             <Collapsible title={copy().purchasesTitle} body={copy().purchaseReviewBody} defaultOpen>
               <Show when={effectiveIsAdmin()}>
                 <div class="ledger-actions">
-                  <Button variant="primary" size="sm" onClick={() => setAddPurchaseOpen(true)}>
+                  <Button
+                    variant="primary"
+                    size="sm"
+                    onClick={() => {
+                      const members = dashboard()?.members ?? []
+                      const currency = (dashboard()?.currency as 'USD' | 'GEL') ?? 'GEL'
+                      setNewPurchase({
+                        description: '',
+                        amountMajor: '',
+                        currency,
+                        splitMode: 'equal',
+                        splitInputMode: 'equal',
+                        participants: members.map((m) => ({
+                          memberId: m.memberId,
+                          included: true,
+                          shareAmountMajor: '',
+                          sharePercentage: ''
+                        }))
+                      })
+                      setAddPurchaseOpen(true)
+                    }}
+                  >
                     <Plus size={14} />
                     {copy().purchaseSaveAction}
                   </Button>
@@ -592,7 +633,12 @@ export default function LedgerRoute() {
             <Button
               variant="primary"
               loading={addingPurchase()}
-              disabled={!newPurchase().description.trim() || !newPurchase().amountMajor.trim()}
+              disabled={
+                !newPurchase().description.trim() ||
+                !newPurchase().amountMajor.trim() ||
+                (newPurchase().splitInputMode !== 'equal' &&
+                  !validatePurchaseDraft(newPurchase()).valid)
+              }
               onClick={() => void handleAddPurchase()}
             >
               {copy().purchaseSaveAction}
@@ -665,6 +711,7 @@ export default function LedgerRoute() {
             <Button
               variant="primary"
               loading={savingPurchase()}
+              disabled={purchaseDraft() ? !validatePurchaseDraft(purchaseDraft()!).valid : false}
               onClick={() => void handleSavePurchase()}
             >
               {savingPurchase() ? copy().savingPurchase : copy().purchaseSaveAction}
diff --git a/packages/application/src/finance-command-service.test.ts b/packages/application/src/finance-command-service.test.ts
index 3f5143b..53ebc8b 100644
--- a/packages/application/src/finance-command-service.test.ts
+++ b/packages/application/src/finance-command-service.test.ts
@@ -752,4 +752,72 @@ describe('createFinanceCommandService', () => {
       }
     ])
   })
+
+  test('generateDashboard should not 500 on legacy malformed custom split purchases (mixed null/explicit shares)', async () => {
+    const repository = new FinanceRepositoryStub()
+    repository.members = [
+      {
+        id: 'alice',
+        telegramUserId: '1',
+        displayName: 'Alice',
+        rentShareWeight: 1,
+        isAdmin: true
+      },
+      {
+        id: 'bob',
+        telegramUserId: '2',
+        displayName: 'Bob',
+        rentShareWeight: 1,
+        isAdmin: false
+      }
+    ]
+    repository.openCycleRecord = {
+      id: 'cycle-2026-03',
+      period: '2026-03',
+      currency: 'GEL'
+    }
+    repository.rentRule = {
+      amountMinor: 70000n,
+      currency: 'USD'
+    }
+    repository.purchases = [
+      {
+        id: 'malformed-purchase-1',
+        payerMemberId: 'alice',
+        amountMinor: 1000n, // Total is 10.00 GEL
+        currency: 'GEL',
+        description: 'Legacy purchase',
+        occurredAt: instantFromIso('2026-03-12T11:00:00.000Z'),
+        splitMode: 'custom_amounts',
+        participants: [
+          {
+            memberId: 'alice',
+            included: true,
+            shareAmountMinor: 1000n // Explicitly Alice takes full 10.00 GEL
+          },
+          {
+            memberId: 'bob',
+            // Missing included: false, and shareAmountMinor is null
+            // This is the malformed data that used to cause 500
+            shareAmountMinor: null
+          }
+        ]
+      }
+    ]
+
+    const service = createService(repository)
+    const dashboard = await service.generateDashboard()
+
+    expect(dashboard).not.toBeNull()
+    const purchase = dashboard?.ledger.find((e) => e.id === 'malformed-purchase-1')
+    expect(purchase?.purchaseSplitMode).toBe('custom_amounts')
+
+    // Bob should be treated as excluded from the settlement calculation
+    const bobLine = dashboard?.members.find((m) => m.memberId === 'bob')
+    expect(bobLine?.purchaseOffset.amountMinor).toBe(0n)
+
+    const aliceLine = dashboard?.members.find((m) => m.memberId === 'alice')
+    // Alice paid 1000n and her share is 1000n -> offset 0n
+    expect(aliceLine?.purchaseOffset.amountMinor).toBe(0n)
+  })
 })
diff --git a/packages/application/src/finance-command-service.ts b/packages/application/src/finance-command-service.ts
index 5ef8491..9ded31d 100644
--- a/packages/application/src/finance-command-service.ts
+++ b/packages/application/src/finance-command-service.ts
@@ -15,6 +15,8 @@ import type {
 import {
   BillingCycleId,
   BillingPeriod,
+  DomainError,
+  DOMAIN_ERROR_CODE,
   MemberId,
   Money,
   PurchaseEntryId,
@@ -867,6 +869,27 @@ export function createFinanceCommandService(
       )
       const currency = parseCurrency(currencyArg, settings.settlementCurrency)
       const amount = Money.fromMajor(amountArg, currency)
+
+      if (split?.mode === 'custom_amounts') {
+        if (split.participants.some((p) => p.shareAmountMajor === undefined)) {
+          throw new DomainError(
+            DOMAIN_ERROR_CODE.INVALID_SETTLEMENT_INPUT,
+            'Purchase custom split must include explicit share amounts for every participant'
+          )
+        }
+
+        const totalMinor = split.participants.reduce(
+          (sum, p) => sum + Money.fromMajor(p.shareAmountMajor!, currency).amountMinor,
+          0n
+        )
+        if (totalMinor !== amount.amountMinor) {
+          throw new DomainError(
+            DOMAIN_ERROR_CODE.INVALID_SETTLEMENT_INPUT,
+            'Purchase custom split must add up to the full amount'
+          )
+        }
+      }
+
       const updated = await repository.updateParsedPurchase({
         purchaseId,
         amountMinor: amount.amountMinor,
diff --git a/packages/application/src/settlement-engine.ts b/packages/application/src/settlement-engine.ts
index 65fb9f2..1fbdbb7 100644
--- a/packages/application/src/settlement-engine.ts
+++ b/packages/application/src/settlement-engine.ts
@@ -259,19 +259,18 @@ export function calculateMonthlySettlement(input: SettlementInput): SettlementRe
     payer.purchasePaid = payer.purchasePaid.add(purchase.amount)
 
     const participants = purchaseParticipantMembers(activeMembers, purchase)
-    const explicitShareAmounts = purchase.participants?.map(
-      (participant) => participant.shareAmount
-    )
 
-    if (explicitShareAmounts && explicitShareAmounts.some((amount) => amount !== undefined)) {
-      if (explicitShareAmounts.some((amount) => amount === undefined)) {
-        throw new DomainError(
-          DOMAIN_ERROR_CODE.INVALID_SETTLEMENT_INPUT,
-          `Purchase custom split must include explicit share amounts for every participant: ${purchase.purchaseId.toString()}`
-        )
-      }
+    // Identify participants with explicit share amounts (lenient read path for legacy data)
+    const explicitShares =
+      purchase.participants
+        ?.filter((p) => p.shareAmount !== undefined)
+        .map((p) => ({
+          memberId: p.memberId,
+          shareAmount: p.shareAmount!
+        })) ?? []
 
-      const shares = explicitShareAmounts as readonly Money[]
+    if (explicitShares.length > 0) {
+      const shares = explicitShares.map((p) => p.shareAmount)
       const shareTotal = sumMoney(shares, currency)
       if (!shareTotal.equals(purchase.amount)) {
         throw new DomainError(
@@ -280,15 +279,13 @@ export function calculateMonthlySettlement(input: SettlementInput): SettlementRe
         )
       }
 
-      for (const [index, member] of participants.entries()) {
-        const state = membersById.get(member.memberId.toString())
+      for (const participant of explicitShares) {
+        const state = membersById.get(participant.memberId.toString())
         if (!state) {
           continue
         }
 
-        state.purchaseSharedCost = state.purchaseSharedCost.add(
-          shares[index] ?? Money.zero(currency)
-        )
+        state.purchaseSharedCost = state.purchaseSharedCost.add(participant.shareAmount)
       }
 
       continue