Vertex is robbing people

These guys called me sometime ago to go make arrangements which i then agreed to initially the person said i owe 6000 which was correct they have been debiting 300 monthly which has now amounted to 3100 my understanding is that im left with 2900 yet they are saying i still owe them 4000 where is the extra 1000 coming from as they telling me my initial debit order is 1250 so my question to them is that why hand me over if im still required to pay the instalment i had with service provider remembering that i couldnt pay that 1250 due to the fact that i was not in a good position what vertex is now telling me was never told when we initially made this arrangement they saying they are charging me interest monthly which does not sit well im about to cancel the arrangement we made and not pay at all as these people are robbing im taking this matter to NCR