Mr Price - Misrepresentation discount coupon - bad service

I am extremely disappointed by the service received from Mr Price Beacon Bay Retail Park, East London branch today! I received an sms with a discount coupon code to the value of R150 a few weeks ago. I went to this branch today to purchase a pair of sliders (size 11) for my son and used the coupon code against this purchase. Got home and when my son fit the shoes on it was too tight. I then took the shoes back to the branch less than one hour later, and was unable to exchange it for a bigger size, as the sliders only go up to a size 11. I then took another pair of female shoes for the same value, and requested the cashier to exchange it for me due to them not having a size 12 in the same slider. The cashier then told me that I only get a R34 discount left from the original coupon discount and that Mr P charged R115 back onto my account...no explanation why or how the process works...the cashier simply said "That's how the system works!". I was upset at the time, as the coupon process was not properly explained to me. I then had to charge the female pair shoes to my account and only received a R34 discount. So in essence, I lost out on getting the other R115 discount, which I feel is extremely unfair, as their male slider sizes only goes up to a size 11!! Is this now my fault that they did not have a bigger size??? Why did the cashier not explain the process properly when I first used the coupon code and then returned the sliders??! I feel that that I need to be compensated for the other R115 due to the inconvenience caused. This is not how you are supposed to do business. I have been an account holder for many years and this was a very disappointing experience! 😔