I would just give the money back or a new key. Sure it was out of your control but you do owe him a game.
Damn you guys! I was hoping for an answer that would benefit myselfIf his key was revoked and he did not redeem it on time, I would give him the new key.
It sucks for you (you miss a freebie which you did not ask for), but it would suck more for him otherwise (he would be missing what he paid for).
If he managed to redeem the key and now asks for the new one, then it is a new trade since the first trade was fulfilled by both sides.
But I'll until the day before the full reveal to see what Humble does