If you are willing to give him a refund because you agree it is the right thing to do then just approve the request when he sends it. If you refuse the request then that will start a dispute. If nothing is solved in the dispute with the mediator then it can be escalated to arbitration.
I don't know the specifics of your situation but I personally would do whatever I reasonably can to avoid any of that, try to talk to your client, and come to an agreement.
