Anyone has an "out" by not honoring a contract. But there's the penalty, which is a potential lawsuit or payment. The HCIW contract specifically spells out the conditions of the contract, which is the school must make him the coach by X date. If they don't, they pay. Hop wouldn't be entitled to the full value of the contract, since he wouldn't be coaching the team, which is what the full value pays for. He'd be entitled to the the penalty.
In Jimbo Fisher's case, he would have owed FSU the same $5MM buyout, had he left earlier than Jan 1, 2011.