charged incorrect amount on product and not refund

On the 30th of december I purchased some stuff from jet,they charged me a higher price than the selling price on the product,when I asked for a refund they said that it would be in a form of my credit on my card,however when I did decide to use my credi the saleslady tells me that I don not hav any credit,how is that possible if my refund was credited onto my card,further more when I called the head office the agent I spoke to said that they did not credit me my money back neither is there anything on the system despite me been told I've been rewarded.I'm really disappointed with their service,I've been ****** also with my account balance,I've been paying my account up to date but the balance hasn't dropped from the time I used my credit,really illiterate staff as well!