Voucher used by someone else and no assistance from MR Price home! Disgrace!!!

In September I purchased a voucher for my mom for her birthday. On Wednesday she went to Mr price home to purchase a few items. When she wanted to use the voucher the lady at the till said the voucher is showing a negative balance of R500. My mom then indicated that this is impossible as she did not use the voucher. The lady then phoned the help desk. After 3 hours they informed us that the voucher has been used online to purchase a phone. We indicated that this had not been us and asked the lady to at least look at the email address etc however she was not interested in assisting us. Now I waisted R500 purchasing an voucher for my mom which has now been used by someone else and Mr Price home is not willing to assist us in the matter. ABSOLUTELY RIDICULOUS!!!! NEVER WILL I EVER MAKE USE OF THIS SHOP AGAIN.