extremely rude supervisor

i bought an item yesterday code *** (lime colour size 14) at gateway , went to exchange item at cornubia woolworths i couldnt the same colour but black in the size i want to exchange, the item is currently on sale online and i purchased it for the sale amount at gateway only to be told by a rude unprofessional supervisor yvonne Naicker that she cannot exchange the item cos its not onsale despite me informing her that its on sale for all colours on line even having to go on line and redeem the pictures. she thereafter rudely spoke to cashier name Tapi in-front of me a a customer, the question is why is it on special online , gateway sold it for sale price but cornubia refuses to do an exchange. Yvonne seriously doesn't have people s****s and doesn't have ethics to speak and interact with customers. she is absolutely rude and needs to be dealth with accordingly.