This is kind of a bizarre thread. I don't understand about pissing off the dealer. Is he that big of a jerk? If telling him you want a 5 foot cutter upsets him, then perhaps you should go elsewhere. I just don't think it would. Tell him the guys on the forum says the L2501 will run a 5 footer just fine.
You have the quote. Personally, I would not buy a cutter from the dealer and deduct the price of the cutter. If you are receiving a discount by adding an implement, get something else you know you will use. Like Bucktail said, there are ton of 5 foot cutters to be had on the cheap around here. 6 footers, not so much.
I would buy the tractor and buy a used cutter and save some money. A 4 foot cutter isn't that wide. My lawnmower is 48 inches.
Dealers are human too. Often times, they have reasons for saying what they do. It may not necessarily be sinister. My dealer convinced me to not get remotes. What his reason was, who can say. In retrospect, I sort of wished I had, although I don't know if I will ever use them though. If I was really sure I wanted/needed them, I would have said I want them. He most likely would have said OK and added it to my quote.