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This particular aspect of the salary cap can be confusing at times. However, once you break down the regulations, it is quite simple.
Teams that are below the salary cap threshold are able to make trades, regardless of the salary of those involved, yet only as long as the trade doesn’t leave them more than $100,000 above the threshold.
Teams above the salary cap threshold cannot receive more than 125 percent plus $100,000 of the departing salary.
What this means is this: Say for instance a player has a $5 million contract, and his team is over the cap. If he is traded to a team who is also over the cap, neither team can receive more than 125 percent of $5 million plus $100,000 ($6.3 million in total). If he is traded for a player who has a matching salary, there is no issue. If his salary equates to more than the aforementioned calculated amount of $6.3 million, the trade would be void.
The following is not directly relative to the salary cap, but it is noteworthy nonetheless. A free agent signed to a team cannot be traded until December 15 of the year he is signed, or three months after the official signing. The different scenarios determining the ability to trade the player are dependent on which date comes later. For example, if the player is signed on December 14, the mandatory three-month wait period comes into effect, rather than waiting one day.
This is to inhibit the idea of signing players to include in trades. Furthermore, players who have been traded cannot re-join their previous team for a period of time. The player cannot negotiate a buyout of his contract and simply re-sign with his former team. He must wait a period of one year or until July 1 of the year his contract expires.
Additionally, a player received in a trade can be immediately sent elsewhere for another piece. This is most evident in "three-team trades," where teams will trade among themselves so all parties receive their desired pieces. This can only be done in a synchronized deal.
For example, if the Dallas Mavericks were to trade Shawn Marion for a player and they wished to combine said player with another in a separate deal, there is an obligatory 60-day wait period before doing so.