Settlement amount increase after paying almost half the settlement amount

I cancelled My contract with netstar in February and Requested a Settlement amount and I receive an email that informed that i will have to pay R1399.22 after i told them that I will pay end of april they told me that on 15th March the account will be with our debt collectors whereby I can make payment arrangement. and I had a chat with those guys and got told that my account is in arrears and need to pay that first so R750 +- debit order went through and when again I asked for the settlement fee they told me that now the settlement amount went up to 1800 ,and they dont consider the debit order that went through but what they did they increased my settlement amount to R1827.66 and the Agent ( Stanley Goliath) that is handling my Query denies to explain to me why /how did the settlement increase after paying 750. he just tells me that its how the system calculate. how do we then trust people who deceive others for financial gains