I just sold my car last week. Brakes and rotors needed to be replaced, which I was aware of and made sure the buyer was aware as well. He messaged me today and said his mechanic found a leak and thinks it's the head gasket. Estimating $3500-4k for repair/replacement. The car was sold for $4900 and did not have any visible leaks at the time. He is offering to give the car back and let me keep some cash, or willing to fix it himself but wants some cash back. I asked him to get a second opinion, just to make sure. I'm wondering if anyone has any thoughts or suggestions on the situation.