Allow overpay invoice with write-off
Sometimes an invoice is overpay, usually because of currency rate changed in favor of the company.
For now, it is not possible to overpay because the full paid amount is put as payment lines of the invoice. But there is a constraint that prevent to put more than the total amount of the invoice. So the user need to find the proper amount (in other currency) that pay exactly the amount.
I think we should split the payment line to allow to put a paying line as payment_lines and the overpay stay on the customer account (until reconcile with write-off).