Player To Be Named Later
Quite often, you'll read that a player has been traded to another team for "a player to be named later."
There are two restrictions at work here. First, the transaction must be completed within six months.
And second, the player named later can't have played in the same league as the team he's being traded to. That's why the player named later is almost always a minor leaguer.
And what if the teams can't agree on who that player will be? This happens rarely, but if no names are agreed upon initially, the clubs will agree on a price to paid in lieu of a player.
Sometimes, at the time of the deal the team receiving the player will provide the other club a list of minor leaguers, and later the club will have their pick of the players on that list. This list is negotiated at the time of the trade. In recent years, the Minnesota Twins lost Enrique Wilson this way. When it came time for Cleveland to make their choice, the Twins did what they could to "hide" Wilson, but the Indians found him anyway.
Finally, sometimes "Player to be named later" is used to trade players on the Disabled List, since it can be embarrassing for a club to trade for a guy who's on the DL.