[GNC] How to apply overpayment to new invoice?

[Eric H. Bowen]

I was late with my property taxes this year and incurred a penalty. I 
had earlier entered the billed amount as an invoice payable to the tax 
collector. I finally got the funds together to pay the amount, including 
penalty, in full. I entered it as one payment transaction, of course, 
since it was one e-check, and now it's showing up in my "Accounts 
Payable" as two transactions, one for the remaining billed amount (I had 
made partial payments earlier), and a separate transaction for $136.63 
representing the penalty and interest which appears as an unassigned 
pre-payment.

OK so far. Well, now that I know the total penalty I entered another 
invoice for that amount. Fine. Now I want to pay the invoice with the 
pre-payment. I go to the payment screen and the pre-payment shows up 
there. But I can find no way to apply it to the new invoice. How do I 
close this [expletive!] out?