incorrect deduction

last of last month called cell c retention departmet asked how much must a pay for my contract to be cancelled they told me must pay R895.65 paid that money directly to their account and sent retention department proof of payment and they changed me from contract to prepaid as my payment was received to my suprised month later they went to my account and deduct 895.65 which is ***** why would you debit my account while i have terminated my contract with you bare in mind if have reversed that payment and stop your debit oder from my account you think you can do as you please on my account and for more information i have destroyed your sim card am not longer interested on using cell c