I think in this case, you give it back. You cancelled, so you should have refunded at that point. Yes, you offered another day and time but he is within his rights to refuse that, like he has.
He obviously has cold feet and if it were him canceling in the first place, then I would say to keep it. Just send it back. As others have said, you can do this anonymously by going into any branch that his bank belongs to and use the deposit machine - just enter the sort code and account number when prompted.