If he wanted out right away, and NU wasn't sure when it could make the change, that'd be a reason. Spurrier and Anderson are two recent examples of coaches giving away money left on contracts in order to leave when they wanted, as I understand it.
There are other examples of this. As I understand it, in 2003 when Michael Jordan wasn't retained as president of basketball operations for the Wizards, he was offered $10 mil in severance that wasn't contractually guaranteed. But he turned it down. So there's a case of both sides trying to give away $10mil. The Wizards were trying to keep up appearances and Jordan was making a point that it wasn't about the money.