If he is anxious to pay $5500, he is most likely willing to pay $6000. Just go back to him and say you were thinking about it and looked at what you have invested and you need to get $6k out of it. Odds are he will be able to cover the other $500 easily.
Then take the money, grab yourself a beater, and go Corvette shopping mid winter when people are really really struggling to sell and you can get the best price
-Tim