Bad promotion- no thorough explanation of the policy

Purchased items to the value of R650 from Ackerman’s in Oxford Street, East London and was discounted R150. Returned a toughees but they did not have size 6 and I was told that from my refund for the shoe they would deduct the R150, which meant I would no longer have a sufficient amount to purchase the toughees elsewhere, none of this was explained to me when I purchased and I wasn’t given a receipt for this refund, I had to complete a manual form.