I have a customer who is also a vendor. I have paid his bill minus what he owes us. How do I credit his receivable with the shortfall from my payment?

QB can't do it directly with one transaction because it cannot handle a transaction that impacts both AP and AR accounts.

Instead, this is done by creating a fake AP payment and a fake AR payment, which will cancel each other out when recorded against a bank account.

In turn, these transactions are sometimes posted to a "wash" bank account (a bank account on your chart of accounts that does not represent an actual account in the real world) or perhaps to a petty cash "bank" account - if you have an account set up for this purpose.