Refund *******

I bought a boot online in April which I had to return today because it was damaged. When I bought the boot I had a voucher that I used so I got about R150 off. When I returned the boot I was refunded an amount less than the amount that was on the invoice. When I asked I was told that they can't give me my voucher back and are not allowed to refund me the full price. I don't understand this because when I bought the boot it was full price, its was not on sale, the voucher was mine. Now I got back way less than I paid for the boot. The amount I was refunded will not buy me another boot. I really want another boot but I can't buy it because I was refunded less than the value off the boot. Can someone please contact me about this.