Monthly installment run on incorrect date

I recently took a small personal loan from FNB. The credit agreement clearly stipulated the date that the payment should be deducted from my account, but FNB has deducted this 6 days prior to my salary date. When contacting their secure messaging service, I am being told that they can't help me, I need to contact a different division. I am not impressed, you cannot deduct payments on a date that was not authorised.