Charged for not having money in my account, but app showed otherwise.

FNB has charged me for using my card without money. When I queried this, the consultant I spoke to was extremely rude and she told me that I need to manage my money better in a very condescending tone. The problem I have is that firstly, my transactions on FNB have been delaying and my app had been showing that I still have money. Secondly, if I use my card and it has no money in it, I expect it to decline like any other normal bank. If this is then not the case, I atleast expect an apology and a SOLUTION on to prevent this in the future instead of being told I need to learn how to keep track of my expenditure. If one needs to constantly keep track of every cent at every moment, then what is the point of the app and giving us the balance? I feel as though I have been ****** in broad daylight, and this was never mentioned to me when I opened the account. I also have two FNB accounts, but why is that with this one, there is this option? I am extremely upset with FNB, and will be closing my account, and this decision was further motivated by your rude consultants.