******** change of debit order

My mum has a greenlight policy with nedbank. I pay the premium ans requested a change from the 15th of every month to the last day of every month. For April may and June it was debited on the last day . Today I checked my account and I was debited on the 15th I called the call center and they said a request was sent to change it back to the 15th . I request an investigation became I did not confirm this in any way. Today I didn't have enough funds in my account and I was charged penalties , who will be liable for this ? Contact me on ***