Refused refund

Went to auto spares Middelburg yesterday to refund a cambelt we bought and I was informed that I need 2 slips for a refund, the other slip they need has the part number on, my husband found the second slip and went there today only to be told that they have a 7 day refund policy which they could have told me yesterday after seeing the first slip. The belt was unused and still in its original packaging still they refused us a refund.