Lack of Full disclosure

I took a loan out with FNB and my agreement is for my debit order to go off on the 20th of the month. FNB decided to take it upon themselves to deduct the money from my account on 18th. I get paid on 20th regardless of the day. I called and they advised that they would refund the money which has still not been done. I advised that if the 20th falls on a Sunday then they should run the debit order on the Monday. Today (21st), another debit order went off my account. When opening the account I was asked what day my debit order is and I advised the 20th. It was not explained to me that if my pay day falls on a Sunday they would take the money on the Friday. Lack of full disclosure to the me regarding my agreement.