Understand that every player would love to obtain a house/car especially when it is their first car/house. And when they buy their first car/house is normally when such issues arise - especially when they buy it off other players.
In such situations always tell the player "the price will be 125k plus 55k". When the player sees "value 125k, buyout 55k" he feels 'hey this is awesome I get it for cheaper than the original price'. Although you have tried to make yourself clear to him, being a relatively new player he hasn't clearly understood what you meant.
I would advise you to give him back the money. He is new, he made a mistake regarding the understanding of what you said.