the manager did not want to change an order

Yesterday (20 Nov 2015) in the afternoon i went to chicken licken and asked the cashier how much do the 12 wings cost. she said R50 (i thought she was saying R15) and i bought them. upon receiving the slip i saw there was no change as i had produced R50 for payment and i asked her why is there no change she said the wings cost R50 and i explained to her i thought you said R15 i would like to change and order something else. she said unfortunately they dont change orders once you buy an item you cant change. i asked her why not because i am a customer and i have rights to change i am fully protected by the law. she called the manager and the manager did not say anything she only said to me they dont change orders and she left. i explained to the cashier that chicken licken is a franchise and they cant apply their own rules not within the rules of the franchise as a whole. they said although chicken is a franchise but it is owned by different people. i said yes thats true its owned by different people but there are universal rules that apply to the whole chicken licken company no matter who own it.ALL i wanted was to change an incorrect order and order something else and they refused