Avis refuse to PAY my Retention Deposit

My insurer Discovery rented out a car via Avis Witbank. I have since returned the car on the 18 November 2015. My retention deposit has never been returned. I complained on the 4th of December and they said I must wait for 7 days for payment. Yesterday 30 December I called them and they put me on hold and I asked that they call me back since they owe me money. Alas they never called back. Worse of all they double charged me without telling me or sending me a quote. And I have to pay interest on the outstanding amount that they debited from my credit card.