A vendor has an open credit. Instead of applying it to the next invoice, the vendor has sent a check. How do I apply this payment and then clear the credit?

A vendor has an open credit. Instead of applying it to the next invoice, the vendor has sent a check. How do I apply this payment and then clear the credit?
IF VENDOR SENT IN MONIES WITHOUT REFERENCE TO A LIABILITY AND IF THERE IS NO OPEN INVOICE TO BE APPLIED AGAINST, A)EITHER RETURN THE CHECK- B)DEPOSIT CHECK AND SHOW LIABILITY TO VENDOR, C)IF OPEN BALALNCES, APPLY CHECK AGAINST OLDEST BALANCE AND SEND VENDOR STATMENT SHOWING DISPOSTION OF HIS FUNDS IN ANY OF THE ABOVE SCENARIOS.