its explained here:
http://blog.fieldoo.com/2017/02/can-you-get-a-club-if-the-transfer-window-is-closed/
basically:
and this is why Marchisio was released before the window closed:
http://blog.fieldoo.com/2017/02/can-you-get-a-club-if-the-transfer-window-is-closed/
basically:
A player who has mutually agreed with his club on the early termination of the employment relationship before the expiry of the transfer window also falls under this exception and can search for a new club during the season.
If a player is released from their club when the transfer window is closed, they cannot sign for another team until the window reopens as well. A notable case of this being Sol Campbell who in September 2009 was released from Notts County, just after a month from signing on a free transfer. He signed for his former club Arsenal in January 2010, after spending a few months training with the team to maintain his fitness.
