Absa charging interest for changing payments dates by 5days, in a same month.

My wife has a vehicle finance with Absa and has recently lost her job, so now I will be paying for her vehicle but my payment date is later to her's. Absa wants to charge us interest for changing the dates, but it's the same month. It's 5days later than my wife's. You try to stay updated with your payments buy they want to charge you more for changing a date, same month 5days difference.