What is your proof? Did you have someone with you? If you have proof and the rental agency is lying and trying to collect a fee not owed, they may have to:
1- Stop asking for the money;
2- Pay you a statutory penalty;
3- Pay your attorneys.
If you have the proof and all docs, a consumer protection office like mine would evaluate at no charge.
The plaintiff bears the burden of proof that the car was returned the next day. And, video is probably on a loop that is saved for 30 days, perhaps 60 days.
Dispute the charges if applied to your credit card and/or contest the pleadings if they begin a cause of action. Sounds like you have a somewhat convincing argument if you have a witness or some other evidence of your drop off. Good luck.
NOTE: The use of the Internet for communications with the firm or this attorney will not establish an attorney-client relationship and messages containing confidential or time-sensitive information should not be sent.