Nashi, legado de la luna

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

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.