Theft of money!

RA/ Invoice number: E ********** 73

We booked a car online and paid with a card. The amount quoted and paid was R 3,813.00, upon return and requesting the invoice the total amount for the hire was R 3,350.00. We requested that the balance of R 463.00 be refunded back on to the card. We were told the following: "Please be advised that we do not refund unused days/amount if your prepaid as per the terms and conditions. Kindly see point 5 on the attached." Firstly what happens to the money as the invoice amount and the payment amount is not the same. My books cant balance because it is not the same amount. We returned the vehicle and were not charged as per the invoice yet AVIS is refusing to return the balance to us. This is *******!