Unreasonably charged for dent in rear bumper

When I returned a car at OR Tambo International Airport, the Europcar agent noted a small dent in the rear bumper, which I had not seen when taking out the car 24 hours earlier (in the dark airport garage). A few days later, I received an invoice over R5625.

While I accept responsibility for not checking the car properly when taking it out, I strongly object to the invoice received for the following reasons: (1) I am sure that the dent in the rear bumper already existed when I took out the car, as I was extremely careful with the vehicle during the entire 24 hours rental period and never noticed even the slightest contact with any object. In my view, it is therefore impossible for the dent to have occurred during the time I rented the car. (2) Notwithstanding (1), charging R5625 for this small dent is absolutely outrageous and unacceptable. This is in no way justified or proportionate.