@svenjosh I don't think you should've told a story like this. You should've just asked about the industry standard and keep it simple.
While I appreciate good courtesy of dealers going out of their way to accommodate. I would not put anyone on the spot like this. He's spending his time, his gas, and his effort on you. He should get something in return. This is at the end of the day, a business meeting, not a casual friend meeting.
I side with the dealer. There's no such thing as a free lunch. I go out to eat and I get charged 20% tip. That's why I don't go out to eat anymore.