Enterprise Rent a Car Claiming false damage to car when returning to a different location Garland, Texas

In November 2012 I rented a car for my sister & made her additional driver@ Enterprise in McKinney, TX.  3800 S. Hwy 75.  It was a Friday afternoon.  There wasn't a car ready so waited for one to be there.  It was around 5:30 & the Ford Fiesta came in from OH.  Car was dirty they did not wash.  We did a quick walk through & saw nothing but wasn't looking at every spot of car just walked quickly by.  Did make comment about being dirty.  I did the walk through sister was not there yet. 

My sis was using for weekend run to her sons so we didn't make too big of deal of it.  Sister went to Denton, TX parked car there for weekend to watch grandkids. 

My sister took the car back to Garland, TX without me Monday afternoon.  I was not with her but co-workers were.  The representative walked through car with her.  Starting getting down & looking very intently at car & made comments to my sister that there were marks.  My sister said she didn't see them & he said nothing.  She called the McKinney Enterprise to let them know he said that & they told her that locations sometimes do that when you take the car back to another location.  Representative at Garland location said nothing about a claim.  My sister had to fight to get a copy of the receipt I had given her to take with her. 

Received deposit back minus charges about 5 to 7 days later so thought everything was fine.  Over a month later received letter that they were charge us for about $650 worth of damage & sent pics.  I have told I want to dispute it.  They ignored me & tried to file a claim on my insurance & my insurance denied it because I was not the main authorized driver of insurance & did not have permission to rent my sister a car under my insurance.  The co-workers even commented they did not see any marks either on the car.

Now they have sent me to collection agency & I will dispute it & they can take me to court if they want to.  I saw on this rip off report website where they try to do this to people all the time & I think that is wrong.  I have always used Enterprise with my insurance when involved in wrecks & never had this problem with them before & shocked that such a supposed reputable company would try to pull a stunt like this but apparantly they have many times. 

That car was filthy & the tiniest of dings show in the picture Enterprise sent & would have never been able to be seen with the car being dirty.  The car had just come from OH.  It has or had OH plates on it.  I will not pay this & will dispute it till the end if that is what it takes!!!!