Dollar Rent A Car Rip Off

I arrived at the Dollar Rent A Car lot at the Chicago O'Hare Airport on 9/10/08. The agent brought around a '08 Chrysler Sebring. The agent left the driver's side door open and I proceeded to load my family and luggage into the car to drive to my destination of lodging. At this time I opened the driver's side front door (for the first time) which caused a dent in the front panel due to the door being misaligned. I phoned the rental car customer service agent and explained the situation. He reassured me that I wasn't liable for the damage because I didn't cause the door misalignment and therefore I should explain the situation upon return of the rental car at my normal drop of date and time. 1 month later I have received a bill from Dollar Rent A Car claiming that I owe them $654.40 USD of damage. I've spoken to the "Damage Recovery Agent", Connie Mccarty, and her supervisor, Gira, multiple times. They have called me a liar and have repeated that I was responsible for the damage regardless of my claims for any form of arbitration or discussion.