heldout_pool/codereverser__casparser-e3a5f714eb37/solution/patch.diff

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3795655..742cb8f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -89,6 +89,20 @@ parser tuned to their template family.
   positive-units branch of `get_transaction_type`. Acts as a
   self-validating safety net for the entire transaction stream,
   not just the Franklin case.
+- **Stamp-duty allocation on split lots.** When a single purchase
+  lot is consumed across N disposals the `FIFOUnits.sell` re-queue
+  carried the *full* original stamp duty on every remaining slice,
+  so the proportional allocation re-claimed the same paid stamp
+  on each disposal — total stamp claimed grew unboundedly with
+  split depth. (Worked example: 300-unit lot with ₹1.25 stamp,
+  split 100/100/100 → total claimed ₹2.29 vs ₹1.25 paid, an 84%
+  over-claim that further widens for deeper splits.) The lot is
+  now re-queued with the *unallocated remainder* (`purchase_tax -
+  stamp_duty`), so the invariant `Σ(stamp_claimed) == stamp_paid`
+  holds exactly across any number of partial consumptions. Section
+  48 only permits deducting the stamp actually paid as a
+  transfer expense, so the prior behaviour over-stated the
+  deduction on Schedule 112A.
 
 ## 0.9.0 - 2026-05-22
 - Add support for CDSL statements
diff --git a/casparser/analysis/gains.py b/casparser/analysis/gains.py
index 6cdab42..240413e 100644
--- a/casparser/analysis/gains.py
+++ b/casparser/analysis/gains.py
@@ -364,10 +364,15 @@ class FIFOUnits:
 
             pending_units -= units
             if pending_units < 0 and purchase_nav is not None:
-                # Sale is partially matched against the last buy transactions
-                # Re-add the remaining units to the FIFO queue
+                # Sale is partially matched against the last buy transactions.
+                # Re-add the remaining units to the FIFO queue with the
+                # *unallocated* stamp-duty remainder — not the full original.
+                # Otherwise a lot consumed across N disposals would re-claim
+                # the full original stamp on every disposal, over-stating the
+                # transfer-expense deduction on Schedule 112A by a factor
+                # that grows with split depth.
                 self.transactions.appendleft(
-                    (purchase_date, -1 * pending_units, purchase_nav, purchase_tax)
+                    (purchase_date, -1 * pending_units, purchase_nav, purchase_tax - stamp_duty)
                 )