Fix: Round amount_finaid() to nearest cent to unblock percentage-aid payments

esp/esp/accounting/controllers.py

@@ -46,7 +46,7 @@
 from django.db.models import Sum, Q
 from django.utils.text import slugify
 
-from decimal import Decimal
+from decimal import Decimal, ROUND_HALF_UP
 
 
 class BaseAccountingController(object):
@@ -623,7 +623,15 @@ def amount_finaid(self, amount_siblingdiscount=None):
                 discount_aid_amount = (Decimal('0.01') * latest_grant.percent) * (amount_requested - amount_siblingdiscount - aid_amount)
                 aid_amount += discount_aid_amount
 
-        return aid_amount
+        # Round to the nearest cent using ROUND_HALF_UP (.5 rounds up, per
+        # maintainer guidance). Percentage-based grants can produce fractional
+        # cents (e.g. 25% of $47.50 = $11.875), which makes amount_due() a
+        # non-integer number of cents. That causes int(amount_due * 100) in
+        # payonline() to disagree with Decimal(amount_due()) * 100 in
+        # charge_payment(), triggering a spurious "inconsistent_amount" error
+        # that prevents the student from paying online at all.
+        # See: https://github.com/learning-unlimited/ESP-Website/issues/6055
+        return aid_amount.quantize(Decimal('0.01'), rounding=ROUND_HALF_UP)
 
     def amount_donation(self):
         lit = self.donation_lineitemtype()

esp/esp/accounting/tests/test_models.py

@@ -4,7 +4,7 @@
 
 Tests LineItemType, LineItemOptions, Account, Transfer, and FinancialAidGrant models.
 """
-from decimal import Decimal
+from decimal import Decimal, ROUND_HALF_UP
 
 from django.contrib.auth.models import Group
 
@@ -455,6 +455,74 @@ def test_amount_finaid_with_full_grant(self):
         finaid = self.iac.amount_finaid()
         self.assertEqual(finaid, Decimal('50.00'))
 
+    def test_amount_finaid_percentage_rounded_to_cents(self):
+        """Regression test for issue #6055.
+
+        A percentage-based finaid grant on a non-whole-dollar amount must
+        produce a result rounded to the nearest cent (ROUND_HALF_UP), so
+        that amount_due() is always a whole number of cents.
+
+        Exact scenario from the issue: 25% of $47.50 = $11.875.
+        Without rounding, amount_due() = $35.625, causing:
+          - payonline:      int(35.625 * 100)  = 3562
+          - charge_payment: Decimal(35.625)*100 = 3562.5
+        These never match → "inconsistent_amount" error → student can't pay.
+
+        With ROUND_HALF_UP, finaid = $11.88, amount_due() = $35.62, and
+        int(35.62 * 100) == Decimal(35.62) * 100 == 3562.
+        """
+        LineItemType.objects.filter(
+            program=self.program, text='Program admission'
+        ).update(amount_dec=Decimal('47.50'))
+
+        self.iac.set_finaid_params(None, 25)  # 25% discount, no dollar cap
+        finaid = self.iac.amount_finaid()
+
+        # 25% of $47.50 = $11.875 → rounds up to $11.88 with ROUND_HALF_UP
+        self.assertEqual(finaid, Decimal('11.88'))
+        # amount_due must be a whole number of cents
+        due = self.iac.amount_due()
+        self.assertEqual(due, Decimal('35.62'))
+        # The two code paths for computing cents must agree
+        self.assertEqual(int(due * 100), int(Decimal(due) * 100))
+
+    def test_amount_finaid_half_cent_rounds_up(self):
+        """ROUND_HALF_UP: a half-cent aid amount rounds up, not to nearest even.
+
+        Per Will Gearty's comment on issue #6055: '.5 or higher gets rounded
+        up; less than .5 gets rounded down.'
+        """
+        # $47.50 at 25% = $11.875 — this is the exact half-cent boundary.
+        LineItemType.objects.filter(
+            program=self.program, text='Program admission'
+        ).update(amount_dec=Decimal('47.50'))
+
+        self.iac.set_finaid_params(None, 25)
+        finaid = self.iac.amount_finaid()
+        # ROUND_HALF_UP: $11.875 → $11.88 (rounds up, not to $11.88 via ROUND_HALF_EVEN too,
+        # but importantly NOT $11.87 as a truncation or floor would give).
+        self.assertEqual(finaid, Decimal('11.88'))
+
+    def test_amount_finaid_percentage_and_dollar_cap_rounded_to_cents(self):
+        """Regression test for issue #6055 with combined percent + dollar cap.
+
+        When both a dollar-cap and a percentage are applied together, the
+        final amount_finaid() must still be rounded to whole cents.
+        """
+        LineItemType.objects.filter(
+            program=self.program, text='Program admission'
+        ).update(amount_dec=Decimal('47.50'))
+
+        # Dollar cap of $5.00 applied first, then 25% on remainder ($42.50)
+        # 25% of $42.50 = $10.625 → rounds up to $10.63
+        # Total finaid = $5.00 + $10.63 = $15.63
+        self.iac.set_finaid_params(5.00, 25)
+        finaid = self.iac.amount_finaid()
+        self.assertEqual(finaid, finaid.quantize(Decimal('0.01')))
+        due = self.iac.amount_due()
+        self.assertEqual(due, due.quantize(Decimal('0.01')))
+        self.assertEqual(int(due * 100), int(Decimal(due) * 100))
+
     def test_amount_due_with_full_finaid(self):
         self.iac.grant_full_financial_aid()
         self.assertEqual(self.iac.amount_due(), Decimal('0.00'))
@@ -464,6 +532,7 @@ def test_amount_due_with_partial_finaid(self):
         due = self.iac.amount_due()
         self.assertEqual(due, Decimal('30.00'))
 
+
     def test_amount_refunded_zero(self):
         """amount_refunded returns 0 when no refunds exist."""
         self.assertEqual(self.iac.amount_refunded(), Decimal('0'))