Product: Debt collection

I would like to seek your help and file a complaint to the company named Midland Credit Management for failing to acknowledge the payment arrangement made by their authorized Debt Collector company which is Atlantic Credit and Finance Incorporated, located in XXXX, VA. On XX/XX/XXXX, there was a confirmed payment arrangement between me and the said Debt collector company. That time my balance appeared as {$7300.00} and payment arrangement schedule will start as XX/XX/XXXX and will End on XX/XX/XXXX with a monthly payment of {$170.00}. I attached a copy of this payment arrangement. Let it be known, that before the payment arrangement was confirmed, I have made an initial payment of {$180.00}. The letter of confirmation given to me after the payment arrangement was made is stating that once I have completely fulfilled the agreed payment arrangement, I will be released of the obligation completely and that the Debt Collector company acknowledge the discounted balance of {$3500.00} and following that 30 days after the completion of my payment arrangement I should be receiving a pay off letter. I'm never late and never failed to pay every month due to my monthly payment was set-up via automatic debit on my bank account. Let it be known that I have successfully completed the payment arrangement and that I did not receive a Pay off letter from Atlantic Credit and Finance even after 30 days of my payment completion. But instead on the XX/XX/XXXX I received a statement ( see attached ) from Midland Credit and Finance that I have a current Balance of {$3700.00} and that I have a new account # XXXX and that I should ignore the account # XXXX from Atlantic Credit and Finance. It appears that Midland Credit Management failed to investigate thoroughly to gather all the details of my account. In addition to this, I have filed a complaint to Atlantic Credit and Finance back in XX/XX/XXXX since Atlantic Credit and Finance failed to me a correct reflection of my total balance with current discounted balance every time they send me a monthly receipt of my payments. Atlantic Credit and Finance mark my complaint as ID # XXXX. A copy of the complaint and their response is attached. It was stated on their response that my payment arrangement is set to be completed by XX/XX/XXXX. 
But again, upon logging in in Midland Credit Management website, it appears the company created another account # XXXX so that they can still go after me and collect the money that was already been settled based on the the account # XXXX. I have attached the statement of Midland Credit Management have created to make it like a new account # and make it appear that I still owe {$3700.00}. 
This is wrong and deceiving. Account # XXXX must be eliminated and must not be created. I also attached the latest statement from acct # XXXX stating that I do not have no further obligation in this debt. But what is the purpose of creating another account number that shows i still owe {$3700.00}.? What is the meaning of it. I need a closure of this account. 
Another Point is, there is a letter of closure ( attached ) that states that the XXXX Account has been sold to Midland Credit Management, there is supposed to be only 1 valid account to be created.