Is this right or wrong?

Ok, so my cousin just bought a new 99 Ford F-150 with chrome rims, tinted windows, sound systems, and a supercharger for $6,000 and he said the truck was mine as long as I pay him back the 6,000 because he wants to get a Chevy Silverado for himself. So I agreed to pay him about 300 a month until the Ford is fully paid off. Is it wrong of him to run up the mileage on the truck and wear it out while I'm still paying it off and not to mention barely driving it? Because I don't wanna pay the full 6,000 and find out that the truck has over 150K miles and needs tune-ups.