Unauthorized Transaction on Account

My account was debited with an amount of R1263.27 on 8 January 2018. My account had a balance of +/- R300.00. I contacted FNB and told them this was not an authorized transaction on my account and i had completed an unauthorized form and forwarded to FNB. Please note at this time that i did all of these things the amount was sitting under pending on my account. I do not have an overdraft facility, so how did FNB allow this transaction to go off my account. it is now a paid transaction on my account and i have been charged R110.00 for an item paid NO1. I think this is unfair and unacceptable that they have allowed this transaction to go through despite me trying to stop it and also why was it paid even though i had insufficient funds.