Scattering Stroke

Oracle Text

Counter target spell. Clash with an opponent. If you win, at the beginning of your next main phase, you may add an amount of C to your mana pool equal to that spell's converted mana cost. (Each clashing player reveals the top card of his or her library, then puts that card on the top or bottom. A player wins if his or her card had a higher converted mana cost.)

Card Rulings

10/1/2007 You decide whether or not to add that much to your mana pool as the delayed triggered ability resolves at the beginning of your next main phase. You don’t decide before then.
10/1/2007 You can’t add just some of the mana to your mana pool. You either add all the mana to your mana pool or none of it.