Historical Rules · 16 January 2016
Rule 601.2h
Official source text from the 16 January 2016 Rules release
The player pays the total cost in any order. Partial payments are not allowed. Unpayable costs can’t be paid. Example: You cast Altar’s Reap, which costs {1}{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 {B}, not {1}{B}, even though you’re sacrificing the Familiar.