Is a credit issued to a customer a debit or credit to Accounts Receivable?

Accounts Receivable is classified as an Asset. Assets have a normal Debit balance. If you mean to say that the customer has paid off some of the amount in their account, then the amount is listed on the Credit side and in the Debit side of the Cash account. If they have bought supplies on the account (owe you money) then the amount is put into the Debit side.