So my old 04 gp I used to have, the original price the seller (being one of my good friends dads) asked was 4000. Well after I bought it we found out that the brakes needed replacing and the rotors were warped. Along with spark plugs needing replacing, the hubs were way beyond bad along with the sway bar end links/tie rod ends and lower ball joints were shot. It hadn't been driven in several months and was high mileage. He decided since I was a friend we would settle at 2000 instead of the 4000 the title says. So after I paid the 2000 he signed the title over and I traded it in for my current gxp. Well now he wants the rest of the money even though he's signed the title which had a lean on it over to me. To which the dealership I traded it in to has now and has had for 9 months approximately now. Also when it comes to money they are very well off...like I've seen how much they make when I used to work part time with. So now he says he's gonna send a certified letter asking for the rest of the payments. My question is anyone with legal experience what can he actually do?