Hate dealer markups, never paid one yet, almost didnt get an online discount once. Found the truck, Went to the dealer, reviewed the paperwork and oops, in-between all the crap, they "forgot" about the online discount. Turned into a big deal for them and the salesperson. Not for us, after all I had the printout of the truck online.
25 minutes maybe later? They had all new paperwork done. To the salesperson credit he didnt once question our position. It was him and the dealership.
As far as the OP. I dont know what I would do, most likely walk and see what else is out there. Most times its supply and demand and chances are someone else will pay it. You do have more negotiation power a few days to a day or so at the end of the month.
Dealers, Managers, Salespeople are under a lot of pressure to hit sales goals, if lucky and they arent at the goal yet, they will make any deal that they can happen. Many times a salesperson will even take a hit on their commission if they have to, in order to hit a monthly bonus.