As above, I handed back a car about 10 years ago and similar happened.
When I dropped it off, the guy inspected it and said great, sign here. A week later a letter turned up saying I owed them €950 for damage.
I told them to go jump, they chased with the usual threatening letters etc, to no avail.
If you've signed off and acknowledged that damage, then that's all they can get you for.
HTH
EDIT: Looks like your sorted. Happy days