Did you take a picture of the damage? It's always difficult to argue these points after the fact, when it turns into a he-says-she-says thing. If you did cause a scratch, it will either fall into the 'too small to care' category, or the 'minor charge for minor damage' category. If it is the latter, €50 sounds about right, given the ensuing admin. You may disagree, and I suspect either a letter to Hertz follows, or a dispute with your credit card, the latter of which may have a higher deductible. But, as I said, I wasn't there and you don't seem to have photo proof.