Customer overpaid but wants remainder to be held in credit

Hi,

We raised an invoice for £100+VAT.

Customer paid £180 (appears in bank feed) but wants us to hold the overpayment as credit. How do we do this properly? I have tried to create an invoice for £100+VAT however, it won’t let me save it.

I am guessing I need to create a credit for them somehow for £180 and chew this down. However, I don’t want VAT to be in there as we don’t know until he buys something what the VAT split will be.

ignore i have figured it out