King's Ransom

When King's Ransom enters the battlefield, exile target nonland permanent an opponent controls
until King's Ransom leaves the battlefield(That permanent returns under its owner's control.)
During the owner of that permanent's upkeep,
they may pay X mana. If they do sacrifice King's Ransom, and add X mana of any color during
your next main phase. X is equal to the exiled permanent's coverted mana cost plus 4.