Google/search lexis/nexis for "first amendment retaliation".
Obviously there are no cases exactly like this one, but the law is clear as is the possibility, if not likelihood, of damages in the millions of dollars.
Given KState's size and the stupidity it would take for University to take any actions to remove this guy, the damages would likely be huge - as the point is to make it painful. KState could easily manage a low seven-figure claim.
And no, the University cannot start digging into the guys past as that would simply support the idea that they are retaliating because of the tweet.
The good things is that the President and University counsel know this and are looking at other ways to address the situation.