Prior to 2019, players could still be traded following the July 31 Trade Deadline if they first cleared revocable trade waivers. Although trades could be completed after Aug. 31 under the old rules, the last day in August was sometimes colloquially referred to as the "waiver Trade Deadline," as players acquired after that date were ineligible to be added to the postseason roster by their new teams.
As of 2019, the July 31 Trade Deadline is the only trade deadline. Players may still be placed and claimed on outright waivers after July 31, but trades are no longer be permitted after that date. With regards to newly acquired players, the Aug. 31 postseason roster deadline remains in effect.
Under the old system, the player's original club had three options when a waiver claim was placed: It could either work out a standard trade with the claiming club (the two sides had 48 hours to agree to a deal), allow the player -- and all money remaining on his contract -- to go to the claiming club with no return or pull the player back off waivers. A player who was pulled back off waivers could be placed on trade waivers a second time, but at that point the waiver request became irrevocable. If a player passed through waivers unclaimed, he could then be traded to any club, free of restriction (though all 40-man-roster players in the trade had to clear waivers before being dealt).
Examples of Trade Waivers
The Chicago Cubs claimed Cole Hamels from the Philadelphia Phillies in August 2014, but the two sides weren't able to work out a trade in the allotted 48 hours and Hamels was pulled back by the Phillies.
The White Sox claimed Alex Rios off waivers from the Blue Jays in August 2009, and Toronto elected to allow Rios to go to the White Sox without compensation as a means of shedding the nearly $60 million remaining on his contract.
In August 2013, the Pittsburgh Pirates claimed Justin Morneau off waivers from the Minnesota Twins, and the two sides agreed to a trade that sent Morneau to Pittsburgh in exchange for outfielder Alex Presley and a player to be named later.
In one of the most famous August trades in MLB history, Justin Verlander was sent from the Tigers to the Astros for a trio of Minor Leaguers shortly before midnight on Aug. 31, 2017. Verlander had cleared revocable trade waivers earlier in the month, making him eligible to be dealt to any team.