That wouldn't stand legally. The reason that they can get away with the transfer window is that it's not the employment of the player that's restricted, it's the transfer of their registration. Any player is quite free to leave their club, paying whatever compensation is necessary to fulfil their contract, and sign a contract with another club but they won't be able to play in FA sanctioned competitions until their registration has been transferred which is dependent on the 'selling' club agreeing. When you say 'Bolton paid £5m for Mugshot McMonkfish' you're really saying they paid £5m for the transfer of his registration.