NFL franchises don't really require investors. They all make massive profits and are a license to print money.
He may not be selling all of the Browns but I would bet its a controlling stake.
Cleveland media is reporting that, Lerner would only sell if it was a majority stake (90% is what they are saying). Also the interested buyer, Jimmy Haslam, is already a minority owner if another NFL team so it won't make sense for him to switch teams unless it is a growth in ownership.