Posting temporary refund as Invoice Due?

[GoldenROM]

I have a client that initially had a $1000 invoice that was paid via AMEX. The cleint was supposed to only be charged $500 and then pay the balance in Net 30. I had to refund the $500 on their card as there was a brerakdown in communications. How do I deduct that $500 from receivables so that it also shows it due on the AR list by that same client?

Thanks