Not sure that applies to this transfer really. If he signed a contract and left next year it would benefit him and his agent, but not Arsenal. He would get a signing on bonus from Arsenal and a years worth of increased wages. The fee of £20m is the top end for someone his age and I doubt that the fact he has a 3-4 year contract at 30 would mean that they would get more than £20m for him.
They could say that they wouldn't sell him as has 3 years on his contract, but we all know that if a player wants to leave then they will.
Arsenal are unlikely to pay more wages to him for a year than United or City would, and when he moves to one of the more high paying clubs in 2013 (aged 30) he'll be paid less than he would if he left now, due to the transfer fee being closer to his real value, if a team is even willing to meet Arsenal's valuation. If he has a shit season, suffers a few injuries, looks like he's on the decline, nobody is going to be interested.
And as for players leaving if they want to, they don't if no-one interested, and even if they are, clubs can dig their heels in and force the player to stay, see Luka Modric.
EDIT: And by the way, if he does attempt this, try to cost United money to benefit Arsenal's coffers, he can go feck himself. You don't screw over your new employer to try and benefit your old one.
