In 18.0, we dropped the support for the round per line rounding method in Mexico. 

However, we didn't think about the following scenario:

- a user creates an invoice in 17.0 in round-per-line mode, and sends it to the Mexican government without problem
- they then upgrade to 18.0
- a while later, they receive payment for the invoice. They then try to send the payment CFDI to the Mexican government. 

The payment CFDI is a complicated beast because it needs to retrieve values from the invoice the payment was reconciled with. Since the amounts in the invoice are slightly different from what they should be in round globally, the payment CFDI gets incorrectly generated and gets rejected by the Mexican government.

So we need a temporary fix that allows the values of the payment CFDI to be adjusted according to the values of the invoice, which works even if the invoice used round per line.

This fix is inspired by the existing solution in version 17: https://github.com/odoo/enterprise/pull/68319
and ensures that the "importe" amount is properly calculated.

opw-4537931
opw-4536649
opw-4528785
opw-4490806