I’m not really the one being addressed, but it doesn’t really matter. He could even write you a separate invoice for it. Surely in his life as a craftsman he often gets the order to do this or that. Who repairs the sink, for example, of the grandmother living alone, if not the plumber. With every task he gets paid for, he carries a responsibility; that’s why he calls himself a specialist in construction. You pay him separately for that, as if it were any other random construction site. Sorry, the guy is really weird. If he worries about warranty while getting paid at the same time, I would think about a career change if I were him. Just ask the owner..... that sounds like: customer threatens with order. Or you ask a carpenter or bricklayer, kitchen builder, electrician, or similar who still comes to you; I’m really stunned. Good luck!