pcs - Write on a piece of paper " I agree to pay you $xxx on (date)" and have him sign it. That will mean about as much as anything else you could write up without a lawyer. The power of any contract is being able to enforce it through a judgement, then being able to collect. I have a written agreement with my future son-in-law that I wrote up. Although it's for several thousands of dollars, as a practical matter if he were to skip out on it then it would cost more to collect than the note is for. I just wrote off over $2000 that a defunct business owes me because it would cost too much in legal fees.
You either trust him or you don't.