(diary to be named later)

With the recent slew of trades left and right by the A's for (insert player / cash to be determined later), I was curious of the league mandates for such strange ambiguous stipulations in a trade. I mean, typically you have your straight player x for player y trade. But in scenarios where it is (insert player / cash to be determined later), how is it dertemined that Jack Cust is worth only cash considerations, and not say a Daric Barton? Or how is Denforia (who hasn't played one bit), worth someone like McBeth, a wonder prospect in the minors. Does MLB / MLBPA have any stipulations as to who/what amount/etc. has to be settled by what date? Just curious how it works out.