Item Unpaid no funds I don't even know.

FNB charged a whole R200 items unpaid no funds fee, when I ask what is it for they tell me its for a purchase I made. I bought an item and the money was reserved immediately from my available funds. 5 days later when I check they have charged me, telling me the store took the money when I had no money in my account. How come because the funds were deducted immediately from available balance? the money was sitting there on pending, what exactly am I being charged for? I want my money back please this is day light *******. Stop charging us for no reason please. According to your fees, you charge R8 per declined transaction, if its more then 9 times that's only when you charge R100. And this wasn't even a transaction I initiated when I had no money in my account. Stop doing this please. i had money when I bought this item.