I bought sandals on the 4th of December using a card so when i went to return then yesterday i told them i lost the slip but i used a card to pay the lady said original slip is need yet i only wanted exchange

I bought sandals on the 4th of December using a card so when i went to return then yesterday i told them i lost the slip but i used a card to pay the lady said original slip is need yet i only wanted exchange.My sons shoe are small and he was suppose to wear them.So what must i do with the small shoes or i must lose the money i bought the shoes with and im currently not working,don't have money to buy another shoes i only wanted exchange that's all and the lady was very rude.