Spawnbroker

Creature - Human Wizard 1/1, 2U (3)
When Spawnbroker enters the battlefield, you may exchange control of target creature you control and target creature with power less than or equal to that creature's power an opponent controls.¹
The trick isn't setting up the bad trade. It's making each side think it got the better deal.
Illus. Wayne England

Gatherer Card Rulings [Nov 2009]

• 10/1/2005: If either target becomes illegal (say, if a creature's power has changed such that the creature you control now has less power than the other creature), then the exchange won't happen.

Format Legality	Legal in Vintage, Legacy, Extended, MTGO Classic Banned in Standard
Printings (1/9)	Ravnica: City of Guilds (Rare)