Unauthorised debit orders and rude consultant

Worst service and attitude I've ever had from your billing department. I made a manual payment on the 20th August for my August invoice- my debit is the 25th. To my surprise, you attempted a debit in my account even after the manual payment reflected on my statement and I was charged an unpaid fee by my bank. Today, you debit me for "the unpaid" amount (mind you, you received my August payment). After I enquire with your billing department, I was advised that this amount is for September..... Why is my collection date changed now all of a sudden???? I was met with serious attitude from your staff and of course I relatiated with vulgar language as I'm being made an idiot. My September debit is only due on the 25th. Who gives you the right to collect whenever you feel like? I need a refund of that amount collected today with immediate effect. I cannot wait for this contract to end.