Can a car dealer demand additional money after the contract for the car has been signed by both dealer and customer?

Hello, I purchased a new 2014 Honda Accord from Brandon Honda in Tampa, FL. I explained to the dealer that I will acquire a pre-approved check from Navy Federal Credit Union to pay for the vehicle. The dealer told me if I finance with them that I wouldn't need any cash upfront as my military discount of $500 would cover the down payment. We talked and he worked some figures, but I still decided to go with NFCU for financing the vehicle. I turned over the NFCU check to the financial guy, we both signed the contracts and I drove home the car. A week later, I get a call from the financial guy stating that since I went with NFCU and not Honda for financing, they need me to pay back the $500 military discount used as down payment to keep the financial paperwork on the contract the same. The financial guy made a mistake of not taking out the $500 discount when he collected the check, signed all legal paperwork with me and yet I have to pay for his mistake? What legal grounds can I stand on?