car rental issue

I rented a car from SIXT a month ago. When I picked the car, I checked it, didn't notice anything wrong. So I drove it out of the rental garage. Right after I drove it for a short amount of time like less than hour, I noticed there is a small scratch on the front window (you can only see it from the inside). I called the company immediately and told them this. They directed me to drive it back and switched me to a new car, and told me "don't worry about this." After a month, I received emails saying I am responsible for the damage.

I already called and told them and disputed this. Is there a better way to handle this? I really didn't do anything that cause the damage.