If there are no Monsters currently fighting (by, say, playing
Pollymorph Potion on it, or using a Wizard's charm spell on it), you can't play
Transferral Potion. If someone uses
Wandering Monster to add another Monster after the fact, or if another Monster was already part of the combat, then
Transferral Potion may be played and the Treasure from the Monster that was removed would go to the player the combat was transferred to, assuming he wins the combat. This was already announced in this
thread.