Hello,

Yes I have a solution, it could be better but it works. Instead of opening the sales invoice and clic on button "Register payment", go directly into Accounting->Sales->Payments. Create a new payment for the customer and validate. If you generate the due payments report, you will see the amount already paid. When the customer will have paid the complet amount, you only have to go into the invoice and associate the payment with the invoice. Let me know if I am not enough clear.

Thanks