Buster Sword

• WOTC Official Reminder Textofficial rules
  Equip — Equip [cost] ([cost]: Attach to target creature you control. Equip only as a sorcery.)

• WWizards of the Coast2025-06-06
  Official Ruling
  If you cast a spell "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.
• WWizards of the Coast2025-06-06
  Official Ruling
  If a spell you cast has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
• WWizards of the Coast2025-06-06
  Official Ruling
  You choose whether to cast a spell from among the cards in your hand as Buster Sword's middle ability resolves. You can't wait to cast one later in the turn. Timing restrictions based on the card's types are ignored.

Rulings via Scryfall