Early debit of contract resulting in unpaid fee

when i took out the mobile contract i informed Mr P Mobile that i got paid ont he 20th of every month - In June the 20th was on a Saturday so Mr P Mobile decided tro debit me a DAY BEFORE I GOT PAID and my back honoured the payment and charged me R90 - HOW CAN I PAY WHEN I HAVENT RECIEVED MY SALARY!!! <br> I would like my debit order to be moved to the very very last day of the month and i would like a refund of the R90 hon fee that i have had to incurr due to them taking my money early - i have always paid by debit order and given them no just cause to take the moneh early!!!