You can't receive a player who was signed and traded if it would put you over the apron. You can send out a player who was signed and traded. If you couldn't, the Nets wouldn't have been able to S&T Bogans to the Celtics in the Pierce/Garnett deal.
So the Knicks can sign and trade Carmelo Anthony, but can't receive a player who is S&T'ed if that is needed to balance salaries.