[FIX] account: switch to refund an invoice with downpayment

If you create a Downpayment invoice from a Sale Order with a fixed
amount bigger than the amount of the SO, when you create the regular
invoice it creates a credit note but all lines are "positive" when only
the downpayment line should be.

Based the implementation existing before d8d47f9ff8
https://github.com/odoo/odoo/blob/3363e55cac2a121c29842a0b2463f940bdd866a4/addons/account/models/account_move.py#L3339

X-original-commit: 5432809bbb80c539ba11fb9aa6973669c30f5827
Part-of: odoo/odoo#101994

addons/account/models/account_move.py

@@ -3378,15 +3378,14 @@ class AccountMove(models.Model):
                 'partner_bank_id': False,
                 'currency_id': move.currency_id.id,
             })
-            move.write({
-                'line_ids': [
-                    Command.update(line.id, {
-                        'quantity': -line.quantity,
-                    })
-                    for line in move.line_ids
-                    if line.display_type == 'product' and line.quantity < 0
-                ]
-            })
+            if move.amount_total < 0:
+                move.write({
+                    'line_ids': [
+                        Command.update(line.id, {'quantity': -line.quantity})
+                        for line in move.line_ids
+                        if line.display_type == 'product'
+                    ]
+                })
 
     def action_register_payment(self):
         ''' Open the account.payment.register wizard to pay the selected journal entries.