From MLBTR
Teams can still deal after the July 31st trade deadline passes. This MLBTR Glossary entry explains how trading in August works:
* Teams have to pass players through revocable waivers to trade them after the July 31st deadline.
* Players acquired after August 31st can't play in the postseason.
* Teams will often put most of their players on waivers to determine interest, since they don't have to give up every player who's claimed.
* Unclaimed players can be traded to any club in August (or even September).
* Teams don't have to trade players who are claimed. They can hand the player and his salary over for nothing.
* If a team places a player on waivers after he is claimed, but not traded, the team loses the right to pull its player back. In other words, the waivers are revocable at first, but not revocable afterwards.
* Clubs have two days to deal claimed players, but they can only negotiate a trade with one team.
* If only one team claims a player, he can only be dealt to that team.
* If more than one team claims a player, he can only be traded to the team in his league with the worst record.
* If a player's only claimed by teams in the other league, he can only be dealt to the team with the worst record.
* Teams cannot pass players on the disabled list through waivers.