What is the point of reversing money

I am so sick and Tired of FNB, i have an issue with money going out of my account and every time i phone the call center they tell me there is an outstanding transaction where i used my account. Which i know about because i get an sms that tells me that R00.00 is reserved for purchase at ocean basket. If i don't see money in my account 2 days later i phone and they tell me it is that... Can someone tell me WHAT IS THE POINT OF RESERVING THE MONEY. This morning I see the is R652.25 and i was told that the R600.00 was at Oceam basket on the 23/01/15 but that money was reseverd coz now it seems like its deducted twice and where did the R52.25 go to. This happen to me a lot and i am not feed up, if this carries on i would just rather change my bank account because i am not HAPPY.