702.48. Offering

Offering is a static ability that functions while the spell with offering is on the stack. “[Quality] offering” means “As an additional cost to cast this spell, you may sacrifice a [quality] 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.”

You choose which permanent to sacrifice as you make choices for the spell (see rule 601.2b), and you sacrifice that permanent as you pay the total cost (see rule 601.2h).

Generic mana in the sacrificed permanent’s mana cost reduces generic mana in the spell’s total cost. Colored and colorless mana in the sacrificed permanent’s mana cost reduces mana of the same type in spell’s total cost, and any excess reduces that much generic mana in spell’s total cost. (See rule 118.7.)