[Nerftw]

We always play that the wording of the agreement is upheld.

So if Bob says "I will help Joe for 2 treasures my choice", and plays a card that adds treasure, Bob only gets 2 treasures and Joe gets all the rest.

But if Bob says (on a three treasure monster) "If I help, Joe can pick the first treasure and I get the rest.", then Bob gets all the rest, since the agreement is that Joe only got one treasure (the first).

Another variant being if Bob says (again, three treasures) "If I help, Joe picks, than I pick two", then Joe gets it because Bob worded poorly. Bob only said he's pick two, which means the extra treasures go to the normal default player, the one whose turn it is.

It makes for a fun way to try to trick other players by wording your offer carefully. Within reason, of course, but after the first time, everybody listens to the offer carefully. That, and I really don't see any reason to rule it any differently than if another player played a card that added treasure.