Brake Squeaking Noise problems of the 2003 Hyundai Tiburon

I was driving my 03 Hyundai Tiburon gt to dinner when I heard the driver-side rear brake start making a loud squeaking noise. I was in the middle of the road and could stop so I tried to get over but by the time I did the wheel locked up and tore the tire apart. I had it towed to dennis Hyundai and they called me and said that the rear brake locked up due to rust forming on the caliper causing it to not release. They said that since it had not been greased properly that it would not be covered under warranty. I rented a trailer and picked it up the car. The person at dennis said that it only needed brake rotors and pads, that the caliper is fine. I went and bought the parts and repaired the car. In the process of repairing the car we checked and there was plenty of grease on the caliper. So, they lied about the cause of the wheel locking up. The problem is still occurring and they refuse to look at it without me paying them. They said since I did the work on it instead of paying them $600-$700 to do it that they won't be able to tell what happened.