This is not a problem: "The problem is instead of selling his interest back to the partnership and having the partnership then sell it to the new partnership he sold it directly to the new partner."

It doesn't have to run through the entity. This is between two people.

I found these:

https://proconnect.intuit.com/community/partnership/help/how-to-report-a-sale-of-partnership-interes...

https://proconnect.intuit.com/community/proseries-tax-discussions/discussion/partnership-sale/00/414...