Historical Rules · 15 February 2021
Rule 702.47a
Official source text from the 15 February 2021 Rules release
Offering is a static ability that functions while the spell with offering is on the stack. “[Subtype] offering” means “As an additional cost to cast this spell, you may sacrifice a [subtype] permanent. If you chose to pay the additional cost, this spell’s total cost is reduced by the sacrificed permanent’s mana cost, and you may cast this spell any time you could cast an instant.”