So Nebraska is going to voluntarily give up millions of dollars that they are contractually owed?
1) Given his track record, I doubt that ANYONE is going to come in and offer Rhule $10 Million per year - or whatever his current Nebraska pay is (with the possible exception of an uber-desperate Pat Kraft, should it come to that). So it is all likely moot.
2) If someone did, and Rhule wanted to jump ship, what does that look like?
You have an athletic program (Nebraska) who's coach wants to "not be there" anymore - Nebraska could say:
"Nope, you gotta' stay - or pay us $15 million". How is that relationship going to do

? You want to pay $10 million per year for a coach who doesn't want to be there?
Or, if Rhule (really, his potential future employer) says: "I'll give you $5 million as a parting gift - and we part at that"? What would Nebraska do - keep a disgruntled coach, pay him $10 million per year, out of spite? Or take the $5 million and go hire a coach who wants to coach at Nebraska?
Do they possibly haggle back-and-forth? Sure. And maybe Nebraska gets lucky and some idiot AD at another university writes them out a check for $15 million. Maybe Nebraska get's frustrated and says "Just get the *** out - and don't come back". Or, more likely, they settle on something in the middle (but Rhule's camp would be crazy to offer anything close to $15 million)