Horrific Exchange Experience, wanted an additional payment as it was a promotional item, not even 7 days after purchase!

I just received the most horrific experience ever trying to exchange shoes I bought my husband for fathers day. I bought the shoes on the 15th of June 2023 as a gift, and the size was incorrect. I came to exchange it today on 20 June 2023 and wanted to take another pair for my son, however they were rude about it and wanted me to make another payment on the original pair I wanted to exchange - apparently due to the fact that last week the shoes were on promotion and now no longer. I feel this is so pathetic and ********* due to the fact that I might have been the recipient of the gift and wanting to exchange it for a smaller size now all of a sudden I have to pay????? HOW? I asked to speak to the Manager at the store in Mall @ Reds named Sarah (this is where I purchased it as well) and she refused to exchange the shoes if I did not pay in the difference, is she even worth a manager as she could have just discounted the difference, and made another sale to have a happy customer leaving the store, but NO. I often buy from them but with the service I just received I will not support them in future. If the policy for exchange on promotional items differ from the normal exchange policy I would have been happy to pay if they just explained it - but nothing was even discussed relating to the policies just demanding a payment. I also tried calling Head Office to get an explanation on the policies, but they dropped the call before I ever spoke to an agent.