I would check with my processor to see if an exception exists for a credit to be returned. If there isn't an exception, I would try to contact the merchant to see if you can send a check. And if the merchant won't accept it or you cannot reach them, I would close the case. 1005.2(m) states it's not unauthorized if the consumer receives a benefit. I would document your attempts to return the funds on behalf of the cardholder, but if you've done all that, I think you have pretty much exhausted your avenues.
(m) “Unauthorized electronic fund transfer” means an electronic fund transfer from a consumer's account initiated by a person other than the consumer without actual authority to initiate the transfer and from which the consumer receives no benefit.