diff --git a/fusion_accounting_bank_rec/services/__init__.py b/fusion_accounting_bank_rec/services/__init__.py
index 25962b99..aea16c90 100644
--- a/fusion_accounting_bank_rec/services/__init__.py
+++ b/fusion_accounting_bank_rec/services/__init__.py
@@ -1 +1,2 @@
 from . import memo_tokenizer
+from . import exchange_diff
diff --git a/fusion_accounting_bank_rec/services/exchange_diff.py b/fusion_accounting_bank_rec/services/exchange_diff.py
new file mode 100644
index 00000000..a5a865aa
--- /dev/null
+++ b/fusion_accounting_bank_rec/services/exchange_diff.py
@@ -0,0 +1,46 @@
+"""Exchange-difference calculation helper.
+
+Pure-Python FX gain/loss computation. The engine uses this for rapid
+pre-checks; Odoo's account.move._create_exchange_difference_move() is
+invoked separately for the actual GL posting.
+"""
+
+from dataclasses import dataclass
+
+
+@dataclass
+class ExchangeDiffResult:
+    needs_diff_move: bool
+    diff_amount: float           # in company currency; positive = gain, negative = loss
+    line_company_amount: float
+    against_company_amount: float
+
+
+def compute_exchange_diff(*, line_amount, line_currency_code, against_amount,
+                            against_currency_code, line_rate, against_rate) -> ExchangeDiffResult:
+    """Compute whether an exchange-diff move is needed and its magnitude.
+
+    Args:
+        line_amount: Bank line amount in its currency
+        line_currency_code: e.g. 'USD'
+        against_amount: Matched journal item amount in its currency
+        against_currency_code: e.g. 'USD' (or different)
+        line_rate: FX rate (foreign per company currency) at line date
+        against_rate: FX rate at journal item posting date
+
+    Returns:
+        ExchangeDiffResult with needs_diff_move flag and computed diff
+        in company currency (positive = gain, negative = loss).
+    """
+    line_company = line_amount * line_rate
+    against_company = against_amount * against_rate
+
+    diff = line_company - against_company
+    needs_diff = abs(diff) > 0.005  # rounding tolerance
+
+    return ExchangeDiffResult(
+        needs_diff_move=needs_diff,
+        diff_amount=round(diff, 2),
+        line_company_amount=round(line_company, 2),
+        against_company_amount=round(against_company, 2),
+    )
diff --git a/fusion_accounting_bank_rec/tests/__init__.py b/fusion_accounting_bank_rec/tests/__init__.py
index 2769b296..7ef65365 100644
--- a/fusion_accounting_bank_rec/tests/__init__.py
+++ b/fusion_accounting_bank_rec/tests/__init__.py
@@ -1 +1,2 @@
 from . import test_memo_tokenizer
+from . import test_exchange_diff
diff --git a/fusion_accounting_bank_rec/tests/test_exchange_diff.py b/fusion_accounting_bank_rec/tests/test_exchange_diff.py
new file mode 100644
index 00000000..b28e3755
--- /dev/null
+++ b/fusion_accounting_bank_rec/tests/test_exchange_diff.py
@@ -0,0 +1,56 @@
+from odoo.tests.common import TransactionCase, tagged
+from odoo.addons.fusion_accounting_bank_rec.services.exchange_diff import (
+    compute_exchange_diff, ExchangeDiffResult,
+)
+
+
+@tagged('post_install', '-at_install')
+class TestExchangeDiff(TransactionCase):
+
+    def test_no_diff_when_currencies_match_and_rates_match(self):
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='CAD',
+            against_amount=100.00, against_currency_code='CAD',
+            line_rate=1.0, against_rate=1.0,
+        )
+        self.assertFalse(result.needs_diff_move)
+        self.assertEqual(result.diff_amount, 0.0)
+
+    def test_diff_when_rates_differ_same_currency(self):
+        """USD invoice posted at 1.35, USD bank line settled at 1.40 -> diff exists.
+        100 USD at 1.40 = 140 CAD; same at 1.35 = 135 CAD; diff = 5 CAD gain."""
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='USD',
+            against_amount=100.00, against_currency_code='USD',
+            line_rate=1.40, against_rate=1.35,
+        )
+        self.assertTrue(result.needs_diff_move)
+        self.assertAlmostEqual(result.diff_amount, 5.00, places=2)
+
+    def test_diff_negative_when_rate_dropped(self):
+        """USD invoice at 1.40, settled at 1.35 -> loss"""
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='USD',
+            against_amount=100.00, against_currency_code='USD',
+            line_rate=1.35, against_rate=1.40,
+        )
+        self.assertTrue(result.needs_diff_move)
+        self.assertAlmostEqual(result.diff_amount, -5.00, places=2)
+
+    def test_company_amounts_computed_correctly(self):
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='USD',
+            against_amount=100.00, against_currency_code='USD',
+            line_rate=1.40, against_rate=1.35,
+        )
+        self.assertAlmostEqual(result.line_company_amount, 140.00, places=2)
+        self.assertAlmostEqual(result.against_company_amount, 135.00, places=2)
+
+    def test_tolerance_handles_rounding_noise(self):
+        """Tiny FX rounding under 0.005 should NOT trigger a diff move."""
+        result = compute_exchange_diff(
+            line_amount=100.00, line_currency_code='USD',
+            against_amount=100.00, against_currency_code='USD',
+            line_rate=1.40000, against_rate=1.40003,  # 0.003 cent diff
+        )
+        self.assertFalse(result.needs_diff_move)