Generally, an MLB club can trade a player on its MLB Reserve List (40-man roster) and minor league reserve lists at any time. However, there are a few restrictions:
1. A player who signs after being selected in the MLB Rule 4 Draft (First-Year Player Draft) cannot be traded until after the conclusion of the World Series. NOTE: Previously, a player signed after being selected in the MLB Rule 4 Draft could not be traded until the first anniversary of the player signing his first contract.
2. A minor league player eligible for selection in the Rule 5 Draft cannot be added to an MLB Reserve List (40-man roster), traded to another organization, or transferred from one minor league reserve list to another within the same organization, beginning with the filing of MLB & minor league reserve lists on November 20th (or November 19th if November 20th falls on a Saturday or November 18th if November 20th falls on a Sunday) and extending through the completion of the Rule 5 Draft.
3. A player cannot be traded while he is on waivers.
4. A player on an MLB Reserve List (40-man roster) who has accrued at least ten years of MLB Service Time with at least the last five years of MLB Service Time accrued with the same club (a so-called "10/5" player) has an automatic "no trade" right for as long as he remains with that club. The player can waive this right if he so chooses.
5. An Article XX-B MLB free-agent who signs a Major League contract after 11:59 PM (Eastern) on the 5th day after the final game of the World Series has an automatic "no trade" right through June 15th. The player can waive this right, but if he does he can be traded only for cash and/or player contracts with a maximum aggregate value of $50,000. Note that an Article XX-B MLB free-agent who signs a minor league contract after 11:59 PM (Eastern) on the 5th day after the final game of the World Series does NOT receive an automatic "no trade" right, even if the player is later added to the club's MLB Reserve List (40-man roster).
POST-2015 ARTICLE XX-B MLB PLAYERS ON CUBS MLB RESERVE LIST: (updated 5-17-2016)
Trevor Cahill, RHP
Dexter Fowler, OF
Jason Heyward, OF
John Lackey, RHP
Joe Nathan, RHP
Ben Zobrist, INF
6. If a "Player to Be Named Later" (PTBNL) is part of a trade, the PTBNL cannot be on an MLB 25-man roster (MLB Active List) at any time beginning when the trade is executed (filed with the MLB office) up until when the trade is completed. A PTBNL can be a specific player or the PTBNL can be selected from a list of players or a class of players as determined by the clubs when the trade is executed. Beginning in 2015, a player signed after being selected in the MLB Rule 4 Draft (First-Year Player Draft) cannot be a PTBNL in any trade made prior to the conclusion of the World Series. Clubs have six months to agree on a PTBNL, but the clubs can agree (when the trade is executed) on a deadline that is less than six months. A cash payment (typically $50,000 for trades involving players on the 40-man roster) can be substituted for a PTBNL if no agreement can be reached within six months, but the alternative cash payment must be agreeable to both clubs and stated in writing when the trade is executed.
7. Beginning at 4 PM (Eastern) on July 31st and extending through the conclusion of the MLB regular season, Trade Assignment Waivers must be secured before a player on an MLB 40-man roster (or 60-day DL) can be traded. NOTE: Because July 31st falls on a Sunday in 2016, the "non-waiver" trade deadline will be extended one day (to 4 PM Eastern on August 1st) in 2016 (only).
8. In most cases a player on a disabled list can be traded, even if the player is not eligible to be reinstated and/or healthy enough to play. The one exception is if Trade Assignment Waivers must be secured before the player can be traded. In that case (only), the player must be eligible to be reinstated from the Disabled List AND healthy enough to play before the player can be placed on waivers. Otherwise the disabled player would have to remain a "Player to Be Named Later" until either the player is eligible to be reinstated from the Disabled List AND healthy enough to play such that Trade Assignment Waivers can be secured, or until the conclusion of the MLB regular season (if Trade Assignment Waivers are not secured).