Unless the contract is torn up due to breach (disciplinary reasons normally), the player has a contract registered with the NRL, and that's binding. It's done to protect players, so that if they are shoved out the door, their contract $$ are guaranteed. So even if the Titans "release" Hayne from his contract, and another team is not willing to foot the full bill, the Titans have to make up the difference.
Eg, Hayne is on what, $1.2 million? If the Eels only want to pay him minimum (eg, $100k), the Titans have to pay him the remaining $1.1 million. Of course that would be stupid of the Titans, and there would be clubs willing to offer more. So the Titans have some power even in this case - as in, they can release him if he signs with a club that wants to foot half the bill, but they can refuse to release him if clubs only want to pay a minimal amount.