Opinions please

Could approach it like that. But put it back in his lap. Tell him he's right (takes the tension out of the start of the conversation). It's his engine and he's welcome to take it back. Then ask, what the original agreement was? And what the decision was when you recommended stopping? Then tell him to bring what he thinks owes and take his engine. If he wants to know the hours - you have them - so you can tell him. All is square. If he's a dck, you may end up with less cash or value but its gone, and he's the dck. If he squares up - at least you got time paid. If he tries to put it back on you, tell him you already told him, and now its up to him.