Incorrect Installment in credit bureau that they dont want to change

My account of 18000, which I was paying 1500 for was handed over to them. They updated the installment to be same as the amount that I'm owing to credit bureau. Which is very frustrating and does not make sense. I did not even know that my account was handed over to them, I received no communication from them and made no arrangement. I called them and made arrangement, then they sent me the letter. I made a dispute to Transunion and received response that NIMBLE group confirmed installment to be 18000. The lady I made arrangement with, had said that they will make changes when they submit on the 10th. This has not been changed and now I am told to pay before they can change the installment that I did not agree to. Can they please change this?? Transunion advised that I report to Ombudsman which is on my plans incase this is not changed. Why was 18000 updated as installment first place???? Why was I not contacted??