Vehicle Finance department not willing to change my debit order date , due to it been less than 7 days

Hi To whom it may concern , i have recently gotten vehicle finance with Absa , the debit date was for the 21st of each month , but that is not even my pay day , so i was advised that i need to call in an change the debit order date after the 1st installment , the lady at the bank told me closer to the debit order date please phone and have the date change which i called up today , and now Absa is telling me that its too short notice , im explaining to the lady there will be no funds in the account on that day , this will now cause a negative balance to my account , each time a debit order fails im been charged R150 , please can someone assist me in moving my debit order date , please im really desperate .