sin collector can i cast the card he chose

[Tilaki]

Sin collector - can i cast the card he chooses

[Arix]

...you're making stuff up.

No, no you can't.

[DatAsh]

You cannot wait until the ability resolves then respond to it. Think about what you're asking for a second, then ask yourself why they would design a card that doesn't work.

[Tilaki]

sorry I meant to say in case he chooses one of my INSTANTS (i know i wont be able to cast sorcery)

[Arix]

That doesn't change anything. Looking, choosing and exiling are all part of the same ability, and you can't do much of anything in the middle of a spell or ability resolving. By the time you get the chance to cast an instant, the card has already been exiled. If you want to do anything before the card is exiled, you have to do it before your opponent looks at your hand.