[FIX] account: keep invoice currency on journal change

Versions:
---------
- 16.0
- saas-16.1
- saas-16.2
- saas-16.3
- saas-16.4

Steps to reproduce:
-------------------
1. install the purchase and account_accountant modules
2. create a purchase order using a non-default currency;
3. create bill;
4. switch to a journal without set currency.

Issue:
------
Currency of bill automatically changes from the one used
by the order to the company default.

Cause:
------
The `account.move` model only looks at bank statements, journal,
and the company default when computing currency, ignoring any
value that might already be present in the invoice:
```
def _compute_currency_id(self):
    for invoice in self:
        currency = (
            invoice.statement_line_id.foreign_currency_id
            or invoice.journal_id.currency_id
            or invoice.journal_id.company_id.currency_id
        )
        invoice.currency_id = currency
```

Solution:
---------
Try to retrieve currency from `invoice.currency_id` before
falling back on the company default.

opw-3434518

closes odoo/odoo#133867

X-original-commit: 114f328575000fd31f117c46381441d7ec0bca05
Signed-off-by: Levi Siuzdak (sile) <sile@odoo.com>
Signed-off-by: Levi Siuzdak <sile@odoo.com>
Signed-off-by: William André (wan) <wan@odoo.com>

addons/account/models/account_move.py

@@ -892,6 +892,7 @@ class AccountMove(models.Model):
             currency = (
                 invoice.statement_line_id.foreign_currency_id
                 or invoice.journal_id.currency_id
+                or invoice.currency_id
                 or invoice.journal_id.company_id.currency_id
             )
             invoice.currency_id = currency

addons/account/tests/test_account_move_in_invoice.py

@@ -2330,3 +2330,41 @@ class TestAccountMoveInInvoiceOnchanges(AccountTestInvoicingCommon):
                 payment_state=payment_state,
             ):
                 self._assert_payment_move_state(move_type, amount, counterpart_values_list, payment_state)
+
+    def test_onchange_journal_currency(self):
+        """
+        Ensure invoice currency changes on journal change, iff the journal
+        has a currency_id set.
+        """
+
+        chf = self.env.ref('base.CHF')
+        eur = self.env.ref('base.EUR')
+
+        journal_gen_exp = self.env['account.journal'].create({
+            'name': 'Expenses (generic)',
+            'type': 'purchase',
+            'code': 'EXPGEN',
+        })
+        journal_swiss_exp = self.env['account.journal'].create({
+            'name': 'Expenses (CHF)',
+            'type': 'purchase',
+            'code': 'EXPCHF',
+            'currency_id': chf.id,
+        })
+
+        self.invoice.currency_id = eur
+        move_form = Form(self.invoice)
+        invoice = move_form.save()
+        self.assertEqual(invoice.currency_id, eur)
+
+        # No change expected with generic journal
+        move_form.journal_id = journal_gen_exp
+        move_form.save()
+        self.assertEqual(invoice.currency_id, eur,
+                         "Changing to a journal without set currency shouldn't affect invoice currency")
+
+        # Currency should change
+        move_form.journal_id = journal_swiss_exp
+        move_form.save()
+        self.assertEqual(invoice.currency_id, chf,
+                         "Changing to a journal with a set currency should change invoice currency")