Dream Halls
Enchantment
Oracle Text
Rather than pay the mana cost for a spell, its controller may discard a card that shares a color with that spell.Rulings (1)
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Wizards of the CoastThis only replaces the mana cost (the mana in the upper right hand corner of the card). It will not pay additional costs from the card text (such as Buyback) or from other effects. It does not prevent you from paying those additional costs if you want or need to.