diff --git a/app/modules/accounting/accounting_mirror_service.py b/app/modules/accounting/accounting_mirror_service.py
index e387605..a75efc9 100644
--- a/app/modules/accounting/accounting_mirror_service.py
+++ b/app/modules/accounting/accounting_mirror_service.py
@@ -472,57 +472,81 @@ def sundry_creditors_aging(
         if closing_balance <= 0.009:
             continue
 
-        credit_rows = [
-            row for row in rows
-            if str(row.get("dr_cr") or "").upper() == "CR"
-            and _money(row.get("amount")) > 0.009
-        ]
+        # True forward FIFO reconstruction.  The selected-FY opening balance is
+        # the oldest outstanding lot.  Every creditor-increasing movement creates
+        # a new dated lot; every creditor-reducing movement settles the oldest lot
+        # first.  The lots that survive at FY end are therefore the actual FIFO
+        # composition of the closing balance.
+        closing_lots: list[dict[str, Any]] = []
 
-        # FIFO reverse reconstruction.  FIFO settlements clear oldest balances,
-        # so the newest credit lots are the ones that survive at the reporting date.
-        amount_to_allocate = closing_balance
-        lots_newest_first: list[dict[str, Any]] = []
-        for row in reversed(credit_rows):
-            if amount_to_allocate <= 0.009:
-                break
-            source_amount = _money(row.get("amount"))
-            allocated = round(min(source_amount, amount_to_allocate), 2)
-            if allocated <= 0.009:
-                continue
-            lots_newest_first.append({
-                "party_name": party_name,
-                "source": "Voucher",
-                "voucher_date": str(row.get("voucher_date") or ""),
-                "voucher_type": str(row.get("voucher_type") or ""),
-                "voucher_number": str(row.get("voucher_number") or ""),
-                "reference": str(row.get("reference") or ""),
-                "original_credit": source_amount,
-                "remaining": allocated,
-                "outstanding_at_fy_end": allocated,
-                "paid_subsequently": 0.0,
-                "final_payment_date": "",
-            })
-            amount_to_allocate = round(amount_to_allocate - allocated, 2)
-
-        # If the closing cannot be fully represented by dated credit rows available
-        # in this mirror, the residual is a genuine brought-forward component.
-        if amount_to_allocate > 0.009:
-            lots_newest_first.append({
+        if opening_balance > 0.009:
+            closing_lots.append({
                 "party_name": party_name,
                 "source": "Opening / brought forward",
                 "voucher_date": opening_fallback_date.isoformat(),
                 "voucher_type": "Opening",
                 "voucher_number": "",
                 "reference": "",
-                "original_credit": amount_to_allocate,
-                "remaining": amount_to_allocate,
-                "outstanding_at_fy_end": amount_to_allocate,
+                "original_credit": opening_balance,
+                "remaining": opening_balance,
+                "outstanding_at_fy_end": 0.0,
                 "paid_subsequently": 0.0,
                 "final_payment_date": "",
             })
 
+        def settle_fifo(lots: list[dict[str, Any]], amount: float) -> None:
+            remaining_payment = max(0.0, _money(amount))
+            for lot in lots:
+                if remaining_payment <= 0.009:
+                    break
+                available = max(0.0, _money(lot.get("remaining")))
+                if available <= 0.009:
+                    continue
+                applied = round(min(available, remaining_payment), 2)
+                lot["remaining"] = round(available - applied, 2)
+                remaining_payment = round(remaining_payment - applied, 2)
+
+        for row in fy_rows:
+            side = str(row.get("dr_cr") or "").strip().upper()
+            amount = _money(row.get("amount"))
+            if amount <= 0.009:
+                continue
+            if side == "CR":
+                closing_lots.append({
+                    "party_name": party_name,
+                    "source": "Voucher",
+                    "voucher_date": str(row.get("voucher_date") or ""),
+                    "voucher_type": str(row.get("voucher_type") or ""),
+                    "voucher_number": str(row.get("voucher_number") or ""),
+                    "reference": str(row.get("reference") or ""),
+                    "original_credit": amount,
+                    "remaining": amount,
+                    "outstanding_at_fy_end": 0.0,
+                    "paid_subsequently": 0.0,
+                    "final_payment_date": "",
+                })
+            elif side == "DR":
+                settle_fifo(closing_lots, amount)
+
+        # Remove fully settled lots and freeze the surviving FY-end outstanding
+        # amounts before any optional follow-up-year settlement is applied.
+        closing_lots = [
+            lot for lot in closing_lots
+            if _money(lot.get("remaining")) > 0.009
+        ]
+        for lot in closing_lots:
+            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:
+            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."
+            )
+
         closing_lots = sorted(
-            lots_newest_first,
+            closing_lots,
             key=lambda lot: (
                 _parse_date(lot.get("voucher_date")) or opening_fallback_date,
                 str(lot.get("voucher_number") or ""),
@@ -633,7 +657,7 @@ def sundry_creditors_aging(
         "allocation_basis": (
             "FIFO from dated Accounting Mirror movements; historical closing is "
             "reconstructed from ledger opening plus selected-FY movements, "
-            "and the newest surviving credit lots form the unpaid closing balance"
+            "and debit-side settlements clear the oldest outstanding lots first"
         ),
         "current_mirror": current.get("mirror") or {},
         "follow_up_mirror": later.get("mirror") or {},