I still have hope for you......because he said "yes" a second time before reneging leads me to believe he is not as determined to keep the car as you would think.
You could try getting him to oblige by telling him all the hoops you went through to get the $$ for his car...selling off your beloved cuda, getting financing in place, AND the fact that he said "yes" a second time with a 15% deposit in hand AND a signed contract. After stating the facts, politely ask him if he would reconsider the hell he put you through and to consider meeting up his end of the deal.
Hey, its worth a try. You don't find N96 cudas much nowadays and sometimes its worth the battle. Not a violent one involving money grubbing lawyers or fistfights, but something two mature men can come together and agree upon.
I'd ask to meet him at his residence where the car is, and from there, do a sit-down and see if you can make this deal happen. Oh, bring the 85% remaining cash with you (preferably a cashier's check), and put it on the table, and get everything in writing.
You never know, something good might come out of it.....