Avis/Budget claims that the payment was rejected but cannot furnish any verifiable or concrete evidence thereof, but keep threatening legal

I rented a small car in June 2024 and as usual paid for the rental at the time of collection, together with the customary holding deposit, and did so using one of my bank cards (not cash). I returned the vehicle on time, and the holding deposit was later released.

Almost two years later I start receiving messages to say that I owe Avis (Budget) R4164.91 for the rental because the payment was supposedly rejected. I no longer bank with the same bankers I was with at the time, and I asked for evidence that the payment was rejected, after all, it is odd that they would have allowed me to take the car if the payment was not successful when collecting.

I am given this crazy explanation that the payment does not happen at the time my card is used on the POS device, but that the transaction is only processed later in the back-office, which is where the employee captured the incorrect *** number, and because of this, the payment rejected.

The evidence I am sent is then an unsigned letter from their bank stating there was a charge-back for the unpaid amount. When I call their bank to validate the authenticity of the letter, they (rightly so) say they can't validate it because I am not a client of that bank nor am I a representative for Avis with whom they are authorised to disclose such information. In addition though the person I spoke to confirmed that the story of the back-end process after the POS device is used is *******. The POS device will immediately give an Approved or Declined message and will retain that information on the device for months after the transaction, and then available from the bank thereafter.

It has now been months since Avis started with the threatening messages, and they are getting more frequent and more intense, but I am not receiving any evidence to really show that the payment rejected. Avis is not accepting and responding to the burden of proof that the payment rejected, and is expecting me to go to the banks I was with at the time and check all my accounts to see if the payment went through.

I have tried a multitude of times via mail and on the phone to get through to someone who can give proper evidence, but just get the same response over and over. I am being threatened with legal action but Avis cannot even give me the very evidence which their legal team will require if they were to hand this over for collection.