That isn't leverage. He has to play for the Eagles if he wants to get paid. If he doens't play, not only doesn't he get paid, but has to give money back. That's leverage.
Joe (stay in control of the Cap) Banner, and Andy Reid will never ever be held hostage by a player. They belive in their system, and are confident they can take an average reciever and make him productive with McNabb to buy time in the pocket, and Westbrook to keep the defense honest. The evidence is that they are better with a great wide-out like TO, but still effective even with an average group. The Eagles have let players go before, rather than paying them. It hasn't caught up with them yet.
Look an NE, who's the star wide-out on that offense? With the O-line and Brady, they get it done. The Eagles with McNabb are the same way.