Lost slip no size exchange within a day of purchase

Bought a pair of shoes for my son at Rage Klerksdorp City Mall store. When I got home the shoes where small. The very next day tried to go get a bigger size and had mislayed the slip and they refused to assist without the slip claiming they needed the invoice number of the slip. Surely they have a record of invoice numbers issued on a specific date alongside the price of the shoe. They are just not willing to assist unlike other stores. I now sit with a brand new pair of shoes that don't fit my son.