Iona and paying casting costs

[jazzykat]

In a abyssal persecutor vs. Iona (naming black) show down I was controlling the Iona and was at negative life. My opponent wanted to play a sac a creature for cabal therapy or a sac a creature/destroy a creature (don't know the name of the card). He just wanted to pay the cost as we both agreed he couldn't play the spells with Iona in play.

I told him that I think it is impossible to pay a cost for a spell that he can't cast. Am I correct?

[dorsch]

If you are not allowed to cast a spell, you can not even begin to cast it and can not pay any costs.