Since u can do the exchange why not regund using the same slip

Gooday i bought a dress size 34 abdd not fitting me, too small so i tried to opt for refund seems am having trouble cos i do not have the main purchase slip but i have used my fnb card Qeens park Giyani branch in limpopo can only assist itj refund only of there is an original slip, not the other visa receipt, and only exchange can be done not refund. And i need refund kindly assist. Glenda