Fuel ***** at Avis on vehicle return?

On 23 January 2024, we handed back our rental vehicle to Avis Windhoek at the airport, and prior to handing it in we fully filled the fuel tank at the airport petrol station. Despite this, Avis still charged us for 23.49l petrol they claim they put into the car. We have shown Avis the credit card receipts as well as photos of the cash dash showing the fuel tank full with the kilometer reading on it (which agrees with theirs). These photos are time stamped. Avis, rejects this without explanation. I believe, the Avis and petrol station agents are in collaboration to *****ulently fill other cars (or fuel containers) and then bill this to the last customer associated with that vehicle. I see this is a frequent complain to Avis, yet they show know intent to investigate this and resolve it.