If he is nice, sell it back at its current price after all your costs are added.
If my wife did this to me I would be extra nice to you to try to convince you to sell it back I would gladly offer more if it had sentimental value to me.
If the guy is a prick tell him to prove to you he is the original owner. If he can't prove it tell him you think he is scamming you and YOU will call the COPS on him.