Mann Bracken fiasco

I had a repayment agreement with Mann Bracken LLC, who was the arbitrator for Chase Manhattan. The agreement went through the court system so it is filed in the courthouse. I have been making payments on the settlement for almost 2 years now. Today my monthly payment was returned to me unopened in the mail. There was a message on the letter: "unable to forward, return to sender". I attempted to call the local Mann Bracken office and when I told them who the account was with, Chase, I was immediately transferred to the Chase customer relations office. I had no idea what was going on. As it turns out, Chase is no longer dealing with Mann Bracken LLC or LLP. The reason was not given to me. My account is now being handled by Chase and if I had not made the call today, I would not have know what was going on and I would probably have been going through collections all over again. Mann Bracken never warned me about the change and Chase had no idea about the contract that I have. I had to make arrangements with Chase to finish paying the account off. It was stated to me that they would not be sending me any paperwork with any written notice that Chase is now handling my account. What options do I have with this matter? I want to get this bill paid but now I am worried that I have made the matter worse by not getting any proof that Chase has my account again. How can this happen? What can I do to get proof for my records so that Mann Bracken can not try to collect anything from me?
If anyone has a suggestion, I would greatly appreciate it.