Fix sundry creditors FIFO for debit opening balances

app/modules/accounting/accounting_mirror_service.py

@@ -479,6 +479,16 @@ def sundry_creditors_aging(
         # composition of the closing balance.
         closing_lots: list[dict[str, Any]] = []
 
+        # A Sundry Creditor ledger can legitimately open with either a credit
+        # balance (amount payable) or a debit balance (advance / sales-side net
+        # receivable in the same party ledger).  The earlier FIFO code created
+        # a lot only for a positive opening balance and silently discarded a
+        # debit opening balance.  That inflated the surviving creditor lots by
+        # exactly the unconsumed debit opening.  Preserve both sides: a credit
+        # opening becomes the oldest payable lot; a debit opening becomes a
+        # carry-forward that offsets the earliest future creditor credits.
+        debit_carry = round(max(0.0, -opening_balance), 2)
+
         if opening_balance > 0.009:
             closing_lots.append({
                 "party_name": party_name,
@@ -494,7 +504,13 @@ def sundry_creditors_aging(
                 "final_payment_date": "",
             })
 
-        def settle_fifo(lots: list[dict[str, Any]], amount: float) -> None:
+        def settle_fifo(lots: list[dict[str, Any]], amount: float) -> float:
+            """Settle oldest payable lots and return any excess debit amount.
+
+            Excess debit is not discarded.  In a mixed purchase/sales party
+            ledger it represents an advance / receivable that must offset the
+            next creditor-increasing credit before a new payable lot is born.
+            """
             remaining_payment = max(0.0, _money(amount))
             for lot in lots:
                 if remaining_payment <= 0.009:
@@ -505,6 +521,7 @@ def sundry_creditors_aging(
                 applied = round(min(available, remaining_payment), 2)
                 lot["remaining"] = round(available - applied, 2)
                 remaining_payment = round(remaining_payment - applied, 2)
+            return remaining_payment
 
         for row in fy_rows:
             side = str(row.get("dr_cr") or "").strip().upper()
@@ -512,6 +529,14 @@ def sundry_creditors_aging(
             if amount <= 0.009:
                 continue
             if side == "CR":
+                # Consume any brought-forward / accumulated debit balance first.
+                # Only the residual credit creates a creditor FIFO lot.
+                if debit_carry > 0.009:
+                    offset = round(min(debit_carry, amount), 2)
+                    debit_carry = round(debit_carry - offset, 2)
+                    amount = round(amount - offset, 2)
+                if amount <= 0.009:
+                    continue
                 closing_lots.append({
                     "party_name": party_name,
                     "source": "Voucher",
@@ -526,7 +551,9 @@ def sundry_creditors_aging(
                     "final_payment_date": "",
                 })
             elif side == "DR":
-                settle_fifo(closing_lots, amount)
+                excess_debit = settle_fifo(closing_lots, amount)
+                if excess_debit > 0.009:
+                    debit_carry = round(debit_carry + excess_debit, 2)
 
         # Remove fully settled lots and freeze the surviving FY-end outstanding
         # amounts before any optional follow-up-year settlement is applied.
@@ -538,11 +565,13 @@ def sundry_creditors_aging(
             lot["outstanding_at_fy_end"] = _money(lot.get("remaining"))
 
         fifo_total = round(sum(_money(lot.get("remaining")) for lot in closing_lots), 2)
-        if abs(fifo_total - closing_balance) > 0.01:
+        fifo_net = round(fifo_total - debit_carry, 2)
+        if abs(fifo_net - historical_closing) > 0.01 or abs(fifo_total - closing_balance) > 0.01:
             raise AccountingMirrorError(
                 f"FIFO movement reconstruction does not reconcile for {party_name}: "
-                f"selected-FY closing {closing_balance:.2f}, FIFO lots {fifo_total:.2f}. "
-                "Refresh the Accounting Mirror for the selected FY and retry."
+                f"selected-FY closing {closing_balance:.2f}, FIFO lots {fifo_total:.2f}, "
+                f"debit carry {debit_carry:.2f}. Refresh the Accounting Mirror for the "
+                "selected FY and retry."
             )
 
         closing_lots = sorted(