When "Assign as Payment" is used on a transaction that is already a payment for an invoice, there is no warning or visual feedback that this is the case. Additionally the payment dialog window that opens does not list the invoice the payment was linked to either, so there's no way to redo the same payment.

If you then assign the payment to another invoice in the payment dialog, again no warning will be given you are about to remove the payment from another invoice.

There are a couple of sensible solutions:
- simply prevent the user from using "Assign as payment" on a transaction that is already linked to an invoice
- warn the user s/he's about to reassign a payment and ask if that's what s/he really wants to do

As an extra bonus, if in the second case the user chooses to proceed, present the now unpaid invoice in the list of open invoices as well. (Tricky - because on "Cancel" the payment should not get altered at all"