Rule 702.25a

Buyback appears on some instants and sorceries. It represents two static abilities that function while the spell is on the stack. "Buyback [cost]" means "You may pay an additional [cost] as you cast this spell" and "If the buyback cost was paid, put this spell into its owner's hand instead of into that player's graveyard as it resolves." Paying a spell's buyback cost follows the rules for paying additional costs in rules 601.2b and 601.2e-g.

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