Here is Saban's contract.
http://www.hornsports.com/docs/Nick_Saban_Contract.pdf
There is no buyout provision. All he has to do is give 30 days notice and he walks. he could negotiate an exit that would modify the terms of the contract, but I wonder why UA would be motivated to do that.
The first hint I was looking for that things were happening was a report of a Texas assistant showing up unexpectedly at a game in which a Bama 5* recruit was playing with the back story being that because Texas has plugged some of the leaks the recruit was looking at Texas. Either there needs to be some attention paid to the mechanics of how Saban would disengage from UA and begin to take control at Texas, of you have to accept he will ignore the details contained in his contract and that UA would not take issue with him doing so.
I haven't followed other high profile coaching changes, so I don't know how these issues are usually addressed. Hopefully, someone can chime in on the details.