Imprint — When Isochron Scepter enters, you may exile an instant card with mana value 2 or less from your hand.
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: You may copy the exiled card. If you do, you may cast the copy without paying its mana cost.
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.