Jet atlantis refuse to exchange without slip

I went to jet store in Atlantis to exchange a pair of tekkies that was bought at jet table view over the weekend of the 26th June 2021. The first time was on Sunday 27th, I sent my brother to go exchange the tekkie for a bigger size but they did not have a bigger size. I have now lost the slip and the takkie sustained a coffee stain which can be wiped, but the store manager and cashier says they cannot exchange the tekkies without a slip. Emphasis is on the slip. The shoes cost 179 rand. How can my son wear a shoe that does not fit. All because we lost the slip .