Intention to charge me interests on late payments

I have a loan with Capitec which i took in december 2014 and the contract said they will be debiting money from my account every 25th of every month, but then as the contract was being read to me over the phone i then said No let's make the debit date to be 27th because my payday is usually 27th and i don't wan't to be charged interests on debit orders that have bounced back on the 25th. The lady said to me she cannot change it currently (in december), but in January i can send in a letter from my HR confirming my pay date so they change the date. In January i send in the letter confirming the 27th pay day but they did not respond to me. This week they respond and say they are unable to change the debit date on their system. I'm not sure if their system require rocket science to update one field of a date. Now it's february and our HR confirmed we will be paid on the 27th this month, but my fear is Capitec will try to debit my account on the 25th as they are reluctant to help me so the date can suit both of us. Now they want to suit themselves by charging me interests when there's no money in my account on the 25th. That kind of service is very very bad.