Treasure Keeper

• WWizards of the Coast2020-08-07
  Official Ruling
  If a card in a player's library has in its mana cost, X is considered to be 0.
• WWizards of the Coast2020-08-07
  Official Ruling
  If a spell 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 Coast2020-08-07
  Official Ruling
  If you don't cast the nonland card with mana value 3 or less, it'll be put on the bottom of your library in a random order with the other cards.
• WWizards of the Coast2020-08-07
  Official Ruling
  If your library has no nonland cards in it with mana value 3 or less, you'll reveal all the cards in your library, then put them back in a random order.
• WWizards of the Coast2020-08-07
  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. If the card has any mandatory additional costs, those must be paid to cast the spell.

Rulings via Scryfall