this does not need to be in writing, a verbal contract is a legally binding contract
he gave you the money you said he owed you and left with his prize, at that exact moment it was said and done
he can argue, disagree and even threaten to sue you, but once he speaks to a lawyer, he will blow away like a fart in the wind
again, once he paid you the contract was completed, his only recourse is to take you to court and PROVE you were negligent
if I may ask, how much money are we talking about?
and no I'm not a lawyer but I know the scenario
