Sounds like you got the old tack on when it came to paperwork....gotta read those things.
Also as mentioned, negotiating based on monthly payment is the worst, and exactly what they want.
When I was buying my F150, the guy kept coming at me with a monthly payment. I told first of all I'm not paying $550 a month for a basic truck (in 2008)...and 2nd, I want to know what I'm paying total for the truck, then I will decide if I want to finance it or just pay cash.
They did not like that because it showed the blatant ripoff. The truck went from sticker special of $20,989 to just under $40k out the door. I negotiated that down to $23k OTD, $28k after interest if I did a loan.
They asked if I wanted 0% interest, or a rebate. I asked what's the rate in the loan with the rebate, its 3%, and the rebate was good enough for a chunky down payment, so that's what I did. Payment came out to $325/ mo, 5 year loan, which I paid off early.
Never negotiate by payment, as mentioned, you will get a $450 car for a $550 payment, if you're lucky.
Also my dealer salesman friend said the two biggest deals to them is bodywork and tires. Cosmetic damage or bald rubber is great room for negotiation, on both trade ins or purchases. You should have had your way with them being willing to take the truck with that bumper damage as is.
My biggest negotiation tool is not giving an eff. If the salesmen knows you want that particular vehicle, you are screwed. I told the guy I don't give a crap what truck I get, I just need a full size standard bed, a tow package, and a extra cab. This had them by the balls.