Unauthorised Debit order date!!

This morning I have been debited an installment for my loan account and my payment date is 20th of each month. The collections department starts off by asking me if my account is in arrears without asking for my details 1st. I provided my ID and she confirms that my debit order date is 18th on their system. I ask her who authorised payment for 18th as I get salary on the 20th of each month. She then says but last month you were also debited on the 18th and I tell her because I was contacted by Absa to get consent to debit my account on a Friday if my salary date falls on a weekend. So whoever phoned me must have changed my debit order date. My concern is that my account was debited on a wrong day and my account didnt have suffient funds for the instalment and I know Absa will charge me via bank charges for an unauthirised debit order. I asked for a reference number for the call and I was told there's no reference number and when I ask what proof will I have that they will reverse the debit order and I wont be charged I was told I was not the only one with this complain and they were working on it! Not happy at all with the response and service!