If you scratched the wheel or its trims/alloys etc, then you are liable for it. Scratches are not wear and tear but damage due to standard of driving ( such as scraping along the kerb ). There is nothing that indicates that the damage was not caused during the rental.

Most places in Europe I have seen have had standard rates in a table for damage to the vehicle.

He quoted a rate which was , it seems, lower than the standard table rate and when you argued , simply charged the standard rate as per the table

If you have credit card coverage for rentals, then just let the rental company deal with it - it seems that you are