Took car in for oil change and when I picked it up...

Took car in for oil change and when I picked it up it had a pretty sizeable dent next to the liftgate window WHICH WAS NOT THERE when it was dropped off. It is such a big noticeable dent that I know had to have occurred while on their premises. When asked about it they denied it and called a 2nd person in that I have no idea who she was and she said that she saw it on there when it came in. (NOT TRUE). They gave me an estimate from their body shop for over $600 to fix. I'm at there mercy and have no recourse. NOW WHAT? I think I know my car much better than they do would know if that huge dent was there previously. It is at eye level and could not have gone unnoticed. I would like to see if management really cares enough to follow up on this complaint and do the right thing. I have purchased 3 Escapes, two from that location - never again! AND for the record, I am looking for a new car.