Seems to me that one would first have to prove that a profit potential was lost in order to argue about the difference in prices. If this buyer did not have a history of buying and selling cars for profit, what does "having one slip away" have to do with it?
I think getting the 500 back is sufficient. What bothers me is that this dirtbag supposedly needed money "desperately" for a new business. So desperate that he is apparently using the 500 PLUS the proceeds of the sale.
What kind of business is needing cash like that?
Better get the money before it all goes for his bail.