Rule 601.2g

The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can't be paid. Example: You cast Death Bomb, which costs {3}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to cast. Because a spell's total cost is "locked in" before payments are actually made, you pay {2}{B}, not {3}{B}, even though you're sacrificing the Familiar.

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