The deal
Hawks get: Sheldon Mac, cash considerations
Wizards get: Protected second-round pick
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Washington Wizards: A
Mac suffered a ruptured Achilles in a preseason game, ending his 2017-18 season and effectively guaranteeing his non-guaranteed salary. (Players on non-guaranteed contracts who are waived get paid through the remainder of their contract until they're able to return to the court.)
Because Washington is over the luxury tax, trading Mac was a certainty from the moment he was injured. Doing so saves the team approximately $2 million in tax payments. So why wait this long? By keeping Mac on the roster, the Wizards were able to minimize the amount of money another team would take on, and, therefore, presumably how much cash they had to send Atlanta to cover his salary.
Atlanta Hawks: B-
The Hawks made this trade solely for the cash considerations. They waived Mac as soon as he was acquired and will have an open roster spot to use.