Seek balance in all things—as long as the scales are weighted in your favor.
—Vodalian saying

Mystery Booster 2

Equilibrium

Whenever you cast a creature spell, you may pay {1}. If you do, return target creature to its owner's hand.

In Rath, there are only so many souls to go around.

Whenever you play a creature spell, you may pay o1. If you do, return target creature to its owner's hand.

Whenever you successfully cast a creature spell, you may pay o1 to return target creature to owner's hand.