Mat&May Somerset Mall, R899 bill that I don't know about

They sell nice genuine leather bags I love their bags but I got a shock of my life when I was told that i purchase with the account whereas I never use the account. When I went back to the shop to query I was put on a phone with TFG contact centre agent who told me i must go to police to make an affidavit to say that I never used the account. Now the TFG forensic dpt told me an affidavit is now enough I must go open a.case bcoz they need a case number to investigate. So disappointed to be sent from pillar to post. Can anyone advise me on how to sort this out? How to prove to TFG that I didn't buy from them on the 2nd November 2019?