UPDATE: ESPN.com's Marc Stein reports the Cavs, Celtics and Nets have agreed to a three-team trade that will open a max salary slot for LeBron James. To help get the deal done the Celtics will use the Pierce trade exception.

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Whenever teams over the cap make trades and don't use all of an outgoing players salary to accept an incoming player the result is the generating of a traded player exception. This exception lasts for one-year and remains available as long as the team stays above the salary cap. Each exception can take back a player(s) for up to the amount of the unused salary plus $100,000.