Credit Card Interest Free Charges

I am so angry and disappointed with African bank, I have been having credit card with them and everything was ok and not even one miss payment on my side , recently I received an sms that my card is blocked and I went to the bank whereby they said its a system error and interest free benefit was added to my card which I do not qualify for which was their fault because i did not app**** for that benefit and they will resolve this issue. This is not fair to me because now they say I must pay R10000 which I did not use at all and they said this started in July but why they did not noticed bug in that month why after 3 months and worse how can you pay for the bug that is in their system .I'm very disappointed with this and I cant wait to close this account because maybe this will happen again and who is going to pay for their system error :( the bank you no trust shame ay never