It's not a matter of selling them back.... theoretically. non of the stipulations of the Rights have ever surfaced, but tidbits here and there from sony and fox lead us to beleive the rights are indefinite as long as they continue have another movie in production every 2-3 years. or 3 to 4.... no one knows.
which simply means they can make them forever, as long as they keep making them within the specified time frame, or the rights revert automatically.
And given how hard this one has failed, you would have to be an idiot to think putting more money into a sequel would be the right idea.