unauthorized debit order

Normally I pay my credit card personally over they years by transferring the minimum due into the credit card . This month I did the same and transferred R.3000.Then a day later FNB debit R2900 . So I called and asked for a refund and was told it's impossible and now it results in my money being short . I'm told to wait for the 2.9 to clear then I'll get the money back which will be lesser than that ... So I want a reversal on the 2.9 please.. It's a huge inconvenience on my side.