My dad shows his love through his pocket book. That may sound harsh, but it's the way he is. He is pretty well off, and if he likes to buy things for people. He's not rich, he just is very comfortable (he has a new house, drives new cars, and I don't believe even his underwear come from Target). My dad is not big on saying I Love You. I've come to realize that his way of showing love is to buy things for people. He isn't trying to buy our love, he's just showing us he loves us. It used to bother my DH, but now he has realized that is the way my dad is. If he can do things that are going to help us out, he will. We've never asked for anything from him, and I think that makes a HUGE difference.
So, I say kiss the man. You didn't ask him to do it. Maybe he thought it was a super deal on the car, and he didn't want to see it pass by. Especially since you don't get it until you sell your car, I don't see a problem with it. It will be like you are buying the car from him instead of the person that was selling it.