Unauthorized debit order

An amount of R2500.00 was paid into my account, yesterday afternoon.

I received the notification, accessed my account through the APP and the balance was R2500.00.

A few seconds later a Debi-check/ debit order went off my account for an amount of R1600.00 and some odd rand.

I am not happy as I have a signed an agreement with the creditor for my account to be debited on the 26th of every month, yesterday was only the 13th.

At no point did I make any arrangements for the Creditor to track my account, in order for money to go off as soon as money is paid in.

I cannot reverse this debit order, through the App or Internet banking.

I need assistance for this debit order to be reversed, as it is an unauthorized/ un- authenticated debit order/ Debi check.