How to enter a credit to a Client who prepays credit. His last invoice has already got a credit attached

Hi

I have a client who prepays monthly amounts. In AR I used to put them in a liabilty account and create an invoice with a negative amount which put the credit into this clients account.

I have tried to do as the forum suggests.

Create a return or credit.

I go to Sales

Create a return or credit

Enter the clients name

This brings up his last invoice which I have already entered a credit.

It doesn't let me do anything.

I have tried to create an invoice with a negative amount but can't do that.

I need to get this sorted as I have bank statements waiting to balance. Once I create an invoice which will be later this month presumably I can then match the bank entry to his latest invoice but I need to be able to balance the bank stements and produce accurate cashflow reports etc. as this will be ongoing

Thanks for your help