I have a customer who has paid an invoice twice, I've applied a credit to their account but I obviously can not assign the credit to the same invoice twice. so I leave it as unassigned. I then create a statement to explain to the customer the invoices that are still outstanding and try to show that they have a credit. but when I generate the statement the credit automatically gets applied to the oldest invoices. This doesn't reflect the situation at all and I'd like to be able to show what invoices have been paid and allow the client to specify what invoice they'd like the credit applied to, so it matches their accounting also. and so there is no confusion when I'm chasing payments again in the future.