Beyond frustrated

I made a purchase at a merchant for R159.98 on 08 September 2023 and my running credit card balance was R9970 which meant that I owed the bank R30. On Tuesday 12 September I noticed that my credit card balance is now less than R99, it is now minus R128.82. I contacted the bank several times and they tell me that my balance is in order. At 1 point I was told balance query is being escalated and the escalation came to nought. Obviously someone within fnb is siphoning monies that cannot be traced and the bank is unable to explain that money difference. It I had a balance of R9970 and not it is R9871, where did the R99 go because there is no transaction showing that I was charged for it. This is not the first time I am experiencing this and the bank is never willing to help or at least investigate the complaints. Its very useless to use the chat system. The agents are not helpful. They can only answer certain things. It's so frustrating. What's the point of having a bank that can't help you. Why must I continue to bank this these people? If there was minus 5 stars, that how I would rate FNB when it comes to this problem