Powerbalance is a rare Enchantment card from Modern Horizons 3 (MH3), collector number 335. Current market price: $0.59 (foil $1.80), TCGplayer market prices via Scryfall.
Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, you may cast that card without paying its mana cost if the two spells have the same mana value.