The part that is so unbelievable to me is that the dealer wants this beat up truck detailed for free but yet will be able to get more money out of it on a sale (thanks to you).
People like that are best to stay away from in business deals.
I personally wouldn't do any work for him but if you really wanted to show him what you can do, I would do 1/2 the hood so he can see side by side the before and after in person. Then if he wants the rest of the truck done YOU set the price. Dealing with this guy, it's best to get everything in writing before doing anything
...and that's why I stay far away from dealerships. They're cheap and don't appreciate the quality of work I do. I'm busy enough to have to deal with these jackazzes.