Historical Rules · 3 July 2020
Rule 601.2h
Official source text from the 3 July 2020 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.