Rude supervisor at Spar - Kensington Place

I was at the Spar in Kensington Square this morning and i tried withdrawing R80 a the till but i was told by the cashier that i needed to buy an item from the store instore in order to take out the R80. Now because i had never encountered such an issue at other Spars i asked the supervisor Marlia/Marla which law this is and how come its only at this Spar i'm being told about it and she rudely said her systems are designed that way so i must buy something in order for them to allow me to withdraw. Please can someone shed more light into this because i really am not aware of sich laws. Also, please can i get an explanation as to why Marla is so rude, she couldve explained this politely but instead her tone and choice of words was extremely unbecoming.