I think this is going be a difficult one for you. I don't know what country you are in and what your mother tongue is, but a negotiation with Sixt would probably have been the only way to clear this up. Sixt has now handed this over to a debt collector so I expect they'll not want to talk to you any further. You could try to take Sixt to court and fight this, but it is unlikely to make financial sense to do so.

The problem is that you have no evidence that you didn't make those scratches, and signed a form that confirmed the state of the car was reflected in the original handover slip. I know it's great to say all this in hindsight, but I don't know what else to tell you.

There is a Sixt representative here whom you could PM.