[FIX] account: bank transaction foreign currency compute

The foreign currency should not be computed if it has already been set.
In cases where the transactions were imported with a foreign currency amount, it should not be changed when the user modifies the date.

Fix for
Task-3338560

closes odoo/odoo#122780

X-original-commit: c31af99542493ca10604688dc2782a0aa41f5294
Signed-off-by: Laurent Smet <las@odoo.com>

addons/account/models/account_bank_statement_line.py

@@ -151,9 +151,10 @@ class AccountBankStatementLine(models.Model):
     @api.depends('foreign_currency_id', 'date', 'amount', 'company_id')
     def _compute_amount_currency(self):
         for st_line in self:
-            if not st_line.foreign_currency_id or not st_line.date:
+            if not st_line.foreign_currency_id:
                 st_line.amount_currency = False
-            else:
+            elif st_line.date and not st_line.amount_currency:
+                # only convert if it hasn't been set already
                 st_line.amount_currency = st_line.currency_id._convert(
                     from_amount=st_line.amount,
                     to_currency=st_line.foreign_currency_id,