What Gives Nedbank the Right to Change Dates of Debit Orders

Nedbank sent a sms that they processed a 'Registered Mandate' on my account for a personal loan. Also, this sms states clearly to call the Nedbank Personal Loans on ***. I missed a sms that stated that the loan deduction will occur earlier and I must 'OPT' out before 5 December 2025. I did not see this and now I have a Registered Mandate. Now I made the call, and they can't do anything and my loan deduction will go off my account on 12 December 2025. The reason given by Nedbank; 'People get paid earlier in December and companies notifies Nedbank that salaries will be paid earlier'. Now, my company did not notify Nedbank and NO, I will not be paid earlier. On top of this, I received a similar sms in March/April 2025 that the date will be changed and I chose on that sms that I DO NOT WANT the date changed. Now explain March/April date of change? So. Other companies notifies Nedbank that they will pay salaries earlier and the rest of Nedbank's client portfolio must now abide by it. I understand that business is business, but Really!? What gives Nedbank the right to change dates of debit orders for Nedbank loans or whatever else may be involving Nedbank business.