I understand that they name a player later in the trade, but how can you keep the value the same? Would the Astros get a better player if Montero does good? Did the Braves just not have a player with limited promise that would be equal to Montero? How do you go about naming the player later? Just wait to see a mediocre prospect that looks like he could use a change of scenery and ask? Just say, hey, you remember that Montero I gave you, how bout that light hitting center fielder that strikes out a bunch?

The PTBNL comes from a list/pool of players that both teams have agreed on prior to the trade. It usually has to do with making roster moves, guys clearing waivers, etc

It has nothing to do with value

There can be many considerations.

Clearing Waivers as mentioned. In the old days, a drafted player couldn't be traded until a time... maybe a year...teams could trade "a PTBNL" in order to trade them on paper, but skirt MLB rules.

I've seen it performance based. If a player makes 10GS the team will get a player from Pool A. If he makes 15 GS, they get a player from pool B.

I think most often, it is about lack of familiarity. You may not be scouting another team's lower levels. Hou and Atl both have their A and A+ in the same league, but AA and AAA are in different leagues. In this case, it could be a couple guys the Astros are familiar with from A+ last year have been in AA or AAA this year and Houston wants to see a couple outings this season before pulling the trigger.

Also, I think it used to be (and may still be) you could not trade a player on the IL... so, you had to wait until they came off to move him.