Payment arrangement made, but not honored

I made a payment arrangment with Markhams last month as i could no longer afford the installments of R800.00. I asked if i could rather make arrangements for between R450.00 - R500.00. The lady i spoke to advised that that should be fine and the arrangment was set up. I got paid yesterday and when i went to draw money for my transport today, i found that Markhams had deducted R2420.00. I dont understand why they have taken such a large amount, when i advised that i will be making smaller payments. This was what was agreed upon initially. Once i realized that Makhams had taken all my money, meaning i have R94.00 left in my account, i called the accounts department and spoke to a lady called Julie. She kept saying my account is up to date, not listening to what i was trying to explain to her. Out of decency i made an arrangement, knowing that i could no longer afford the installments monthly. How will i go to work? What will i eat? is there anyone that can stop this debit order and reverse my funds? The service i recieved was utterly putrid. I am so furious. I just got paid, and i have no funds left.