Bad service

I was at a sports scene branch on the 25 may 2023. I had a reward voucher of sR200. So while paying we saw that some of the items I was paying for were giving automatic discounts( which was redbat track tops) and I also bought a pair of all stars that is on sale. Soo before the cashier could process the transaction I asked her to separate the transactions since I wanted to redeem my voucher, and she confidently told me that I would still qualify to redeem the voucher. Soo in total I was supposed to get R400 off, inclusive of the store discount. But immediately after leaving the store I learned that I only got R200 off and I went back to the store and they refused to credit back the items so that we can do a return. What made me angry is that they knew there guys doing maintenance in their store so they where eventually going to switch off their systems if they don't do the credit on time. They said they called their head office and were told it not possible for them to do the return, which i left the items and carried on with my shopping in the mall as they promised to call me back, which they did not. Went back to the store, they still had no answer for me... Their line manager know to be found, then I took my clothes they still promised to call me back the next day which was Friday the 26th up to this day they still didn't call back. This shows they don't value me as a customer