I had a similar situation many years ago buying a car for my Mom. I'm a tough negotiator and I got what I figured, based on my knowledge of what the car was worth, a good price. Well, I brought my Mom into the dealer when the car was ready and the owner pulled me aside and told me they had made a mistake on the price but that he would honor the deal. Basically, cause since I had a signed deal he didn't have much choice. I knew I had done well but it wasn't like he was losing money on the car. More like he just wasn't making as much as he figured he should. She bought the car anyway at the agreed price and I never bought another car from them again. I also to this day have never forgotten how he made me feel. A deal is a deal, and of course, he never showed me any paperwork showing his supposed mistake. Needless to say, he lost a customer for life and I told many people over the years about this and I'm sure it cost him business in the long run. Some people just never learn to overcome their own greed.