Nashi, Moon's Legacy

If you don't want to cast the copy, you can choose not to; the copy ceases to exist the next time state-based actions are performed.

You cast the copy while the ability is resolving and still on the stack. You can't wait to cast it later in the turn.

Because you're paying the spell's costs, if the spell has {X} in its mana cost, you may choose its value as normal.