Unnessacery deductions.

Not sure if this is a new things only seems to be affecting me more recently, I had R423 taken from my account end of July. Reason being as per FNB no funds for purchases made however at the time funds available and came off the account. Feedback was that the purchase were pending and based on when the other banks vendors submit those purchases which from what I saw was quit some time. What I don't 100% agree on is that funds came off the total and its not in my control how long other vendors take to process those purchase how do I still get billed so much.