MTN bad customer service instore

I went to MTN in mall@reds on Saturday to get a price for the Huawei P20 lite phone. I was happy with the price and said I want to purchase it. Mr Van Staden did all the paperwork, copy of my ID etc. Unfortunately I had a limit on my debit card for POS. I quickly went home and went on the internet to change it. I then went back to the store and tried again. Unfortunately, my card was once again declined. The banks were already closed, and therefore I couldn't do anything about the limit for POS. I asked Mr Van Staden to keep it for me, I will come in Sunday morning to pay for it, seeing that all the paperwork was already done. This was closing time for the shop 5pm. On Sunday, I got to the MTN shop at 9:15. He was ignoring me like a stopstreet, trying very hard not to make eye contact with me. When I eventually got to the front, I was told, oh sorry, we just sold that P20 Lite to another customer and that was the last one. After he promised me he will keep it for me till Sunday morning. Needless to say, I was very disappointed. Specially the way I was treated.