FNB robbing people

Hi all, i have a problem with fnb swipes. Example: i have R100 in my account, i purchase an item at a store for R100 then i receive a message saying R100 reserved for purchase then im left with R0 aavailable in my account, now my challenge/problem is in two days i receive a message saying R100 paid to store, but now im having a -R100 available in my accout, so my question is what happened to the reserved R100? Why do i have a minus balance?

I phoned fnb they told me that is because the merchant banked their speed point few days after. The answer they are giving dint make any sense because i had money in my account why now am i owing the bank.