Information withheld upon Purchase

Vodacom phoned me to take out a new contract and offered me a Homemark voucher. When I went to our local Homemark store in Festival Mall, Kempton Park I purchased 2 items with the voucher. When I asked the saleslady if there is a guarantee on the items she told me that I had a 7 day money back guarantee. The one item was not to my satisfaction and when I went it to return the item I was told the the 7 day return does not apply to items bought with a voucher. I feel that this information was not given to me on purchase, they now even say that I cannot even exchange the item for something else. I really feel strongly that they should point this out to customers buying with this kind of voucher.