Exchange for less than the purchase value

I bought items from Earthchild Clothing in Eastgate on the week of 15 to 20 June and they were intended as a gift for my niece. I bought baby boy clothes and when the baby was born it was a girl. I went to get the clothes exchanged and was told they would be exchanged at the sale price and not the full price I paid when making the purchase because the items are now on sale and they could not track my shopping history using my cellphone number as this was not captured at the time of purchase and this is \standard procedure"\"". Can someone explain how this works because it means I'm now donating money to Earthchild? """