Debi Check Lies

I want to reverse an unauthorized debit order and was told system is blocked need to speak to the third party i have the agreement with to cancel the debi check in order for fnb to do a reversal. I spoke to them and they refused. Whys is it that my own bank cant even assist me i have spent over a 100 just in airtime. The debi check mandate on your website does not state that you will block the debit order from being reversed once you accept it. What is happening with all the orther T&C i never agreed to. My account was double deducted but my bank cant assist.

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please can someone please assist or should i open a ***** case what must i do