Unauthorized debit orders

I have a loan with Finchoice. They debited my account 3 days before my agreed pay date and put my account in overdraft. I called my bank to reverse the debit and also subsequently sent an email advising them of their error. They responded by saying that the contract says they can debit my account two business days before. I then went onto the mobi site to amend the debit order date to two days after my pay date to accommodate their 2 day before clause. Two days later they decided to debit me twice. I can't believe all the trouble I am having dealing with their customer care team.