Vaarerk, Gnostic visionary

When Vaarerk, Gnostic visionary enters the battlefield name an instant or sorcery spell.
Whenever you cast the named instant or sorcery
spell you may exile it after it resolves.
Whenever you cast a spell with the same type as
the named spell, it the named spell is exiled, you
may pay {u}{r}. If you do cast the named spell
without paying its mana cost.