Something similar happened to me. I would like to share my story.
I had a dispute with one of my credit card company three years ago. The credit card company sold my account to a collection agency and that collection agency kept chasing me for the balance (the original balance is some fee related to the credit and the rest is the accumulated interest and penalty). I filed dispute with the credit card company and it finally agreed to waive my balance in full in formal business letter. However, it forgot to notify the collection agency as it stated in that letter. About a few months ago, the collection agency called me again. I forwarded the letter to them but they deemed the letter invalid because that credit card company had filed bankruptcy last year. I checked my credit report and found the negative information on it. I then filed complaint with the credit bureau. The credit bureau accepted my story and deleted that negative information from my credit report.
I suggest you may do the same thing.
However, I also have a question:
In my case, even if the credit bureau is already on my side, what will happen if that collection agency keeps chasing me?
Your help is greatly appreciated!