I'm going to reimburse my friend with a game key instead of actual money
Why won't you give your friend actual money?
I am out of money at the moment, and only have the game key.
If your friend is fine with this, then it is ok. But usually reimbursement is done with cash, or in the same method that you received the money.
I am sure he will like the game.
Even if he does, you should give him money. There is no way to make sure that the game key is the same value as the money you owe him.