Processing payments?

[John Whitmore]

GnuCash has gotten itself confused and it's now confusing me. Actually it didn't
really confuse itself it must really have been me that confused it.

So I'm running a business and just tried to process a payment in GnuCash. I
got offered the outstanding invoices and that's where I'm confused. I'm only
starting out so the numbers are low here. Invoice-01 has been paid in full but
the process payment window still offers me that invoice. It's best I recreate
the window's contents. I'm running a plain text mail client so indentation
will be out.

Date      Number        Type        Debit     Credit
07/12/15  Invoice-001   Invoice     1,000
24/12/15                Pre-Payment           1,000

The problem is that the payment on the 24/12/15 was for Invoice-001 and I've
looked at it and it's marked as being for that invoice but for some reason
it's marked as "Pre-Payment" I've no idea where I put that but can't seem to
edit it at all.

Anybody know how to I change that to get these two resolved?