Once you have calmed down, and no doubt I would be beyond furious, I would write a letter to them, explaining your thoughts and feelings, and why you know indoor is better, that this sort of thing keeps you up at night, that you could never forgive yourself, and you will work to find him a new home, etc.

I wouldn't rely on any written contract. That just sets the two of you at odds, I would work to appeal to this person's emotional side, not the legal side.

Thank GOD this kitty is ok, relatively speaking.