Rediculous bank charges

This is the same problem I experience every month. I fail to understand why I should be penalized if the merchants delays to claim from the bank. I used my card on the 24th to purchase petrol for R600 and there was more that sufficient funds in the account. I received a SMS confirming that the transaction was approved at the merchant but now I'm charged R120 for the transaction. Customers cannot be penalized for your systems flaws, sort out your systems and stop penalizing customers. If I have R5000 in my account and I use R600, the money is deducted from account and leaves me with R4400 available balance. Where does that R600 go because is off my account? I'm not a IT expert and I'm not interested in how your system works. When I opened an account with FNB, I was looking for a bank service, not a crush course in IT. I was promised last month that they will put a "limit" or a "flag" to prevent this from happening but here we are again fighting the same battle. I need my R120 reversed, I didn't develop FNB systems and I refuse to be punished for such.