Remove Customer's credit

I have searched the website for an hour now, but the same question seems to have different answers once the details come into place.
Here is what I have:
The customer decided to give me a check in advance, so I can apply it to the future invoices as they occur. She did not use all of it. Her husband is also my customer (they both have businesses and I have them listed as separate accounts). They asked me to apply her remaining credit to his invoice.
I received a payment in full from him (probably, I shouldn't have done that), so his invoice shows as PAID. So, I need her credit to be zeroed out. I'm not sending her a check, nor it's a refund, correct? If I issue a credit memo it doubles her credit. Clearly, that's not what I need :))
Thank you in advance for your help!