Contract debited me without our agreement

Two weeks ago an agent of virgin mobile called me to make payment. We came to an agreement that I will make payment of R500 when I get paid n she promised to send me banking details. To my surprise same night deduction of R500 debit was made which resulted into unpaid cause there was no money in my account n now on the 15th they just decided to take R2 626.62 from my salary without agreement n now I'm left with nothing cause that was my levy n petrol money. Can't they please go back and review our agreement.<br> <br> <br>