Still waiting (4 months) for proof of chargeback

Avis (who bank with Nedbank) claimed that our payment for a rental was reversed and I made another payment.<br> Our bank & Visa denied the chargeback and supp**** us with proof of successful payment.<br> I was put in contact with Selome van Niekerk from Nedbank who have, since May 2015, been unable to provide me with any proof of this chargeback. She keeps referring me to our own bank, and appears to just be passing the buck instead of actually finding out where the money has gone. Surely a bank should have transactional, legal proof of all transactions? Why can Nedbank not supply this proof???