Processing a partial refund from credit memo for a retainer

Hello, I have a retainer from a client and I need to issue a partial refund for a portion of the job that was canceled. The way it’s currently setup, when a client pays a retainer I create a sales receipt and a credit memo and they get applied as services are rendered and the invoice is issued. I need to issue a partial refund to the client from their retainer and hold a portion as deposit for services that have not yet been rendered. When I try to give a refund from the credit memo it doesn't allow me to change the amount . I can issue a check and make it billable and create an invoice but I'm not sure if this is proper way of doing it. Any help would greatly be appreciated