It's possible but what happens often now is the professor is getting paid twice. Paid sabbatical and bonus money for the book. If the University was doing it, they wouldn't pay a person twice for one job. At least that's how my university would view it. Also, I'm not sure if a professor gets much credit from the University toward tenure for writing a book. Because of this, if there is no money involved, I doubt many professor's would do it. They do it for the money only right now.