SL 6.5 - Entering a pre-payment check

I am trying to post a payment to an account as it was paid prior to the invoice being received.  The check is also for more than the invoice, which would result in a credit on the vendor.

Can someone please assist me with this process as I have tried and SL will not allow me to post a check for more than the invoice.  1. I don't have the invoice, 2. When I got the invoice it was for more than the invoice amount..

Please help... the help menus are of no assistance.  Thank you!