Disputing a collection on an account that is current, but the collection agency wants the original debt amount

I have a surgery bill (Original Balance: $2,757) that I have been paying off for the past 2 years with auto-drafted monthly payments. Every month on the 1st a payment was auto-drafted from my checking account. I had the bill almost paid off (Current Balance ~$600.) I made every payment on time (auto-draft), and have a receipt from the doctor’s office for every month. I got a new debit card, and called to ensure the payments on the account would continue. I was informed the surgery center had been sold to a larger health system, and my account was sent to collection. I asked why if my payments were made on time, but was told there is nothing they can do because the company doesn’t exist anymore. I was also informed that all existing accounts were either sent to collection, or to a 3rd party billing company to collect.

I was not sent any information that I need to pay a different company. I was given the collection agency phone number and case number. I called the collection agency, and they informed me I owe the full ORIGINAL BALANCE ($2,757). I asked when it went to collection and they said 08-03-2020. I explained I didn’t understand because the payments have been auto-drafted for years, and the balance is almost paid off. They didn’t care. They said I could either pay the original balance in full ($2,757), or dispute it.

I filed a dispute on Credit Karma, and called TransUnion. I told them I have bank statements of the auto-drafts, and months of receipts from the doctor’s office. It is currently in review.

Is there anything else I can do?

Thank you in advance for any advice.