Originally Posted by Yachtzee
Actually, he already got a lot of his money before he played a down. Signing bonuses are the best kind of guarantee because its money in hand before you've been asked to perform. In contract law, that's called consideration, which means once you take that money, you have a duty to perform the contract. The way NFL contracts are actually constructed, the signing bonus should be considered the real compensation and the so-called "salary" is really a performance bonus.
In the real world, if anyone of us signed a contract paying us up front and refused to perform, we would be sued civilly for breach of contract and could also be charged with theft. On top of that, our employer could even prevent us from working for competitors for the length of the contract.
If NFL players wanted more flexibility in deciding when they can leave a team, they could alway just sign one year contracts and renegotiate every year.
In the real world, agents exist to make sure the actual contract language doesn't say what you say it's saying.
Now, you are correct about one thing -- if a player takes a big signing bonus, he owes the team his best effort over a certain time period. Where you're incorrect is the assumption that this time period is the entire nominal length of the contract. No one in the NFL signs "real" nine-year contracts. The only reason deals are ever that long is because the player is doing the team a favor -- giving the club a longer period to pro-rate the signing bonus so it saves them room under the salary cap in the near term. If you're going to lawyer up and try to cut them off at the knees for going along with that, then expect them to stop doing that.