Restated Discovery Card Balances

My husband have tried to close the Discovery Card account. It is a credit card. Discovery Catd requires 2 days notice to cancel the card. He has placed the notice on Wednesday 5th June - the credit card balance was full i.e. no amount was outstanding to Discovery. By Thursday afternoon the Card was still active but the card balance has reduced by R70 i.e. my husband became liable R70 to Discovery beck the balance reduced without any transaction backing the reduction. He paid the R70 to make sure that the card is closed. On Saturday morning the balance further reduced by R30 again indicating my husband owes Discovery R30 without any transaction going off the card. My husband called Discovery Card to confirm how and why the balance have reduced on the system, the agent could not confirm why and was suggesting that it could be interest on outstanding transactions (I note that nothing was outstanding as per Discovery card balance on Wednesday morning and afternoon, there were no backing transactions to correlate to reduction of the balance and credit card facilities are interest free for 30 days). On Monday, the Discovery Card sent an email to my husband stating that if my husband wants the card to be canceled he needs to pay R30 otherwise Discovery will not close the card. It's not a large amount but there is no guarantee that in the 2 days waiting period to close this card from today there will be no further changes to the balance. I believe that Discovery should review their system and statements to explain to the customers how the balance can possibly change without transactions on the account.