Problem with edited+Trigger enhanced phoenix fire

[Thunderhorn]

My problem is this: I want some dummy to cast an ability for extra effects at the target of my phoenix fire , but the unit does not have the spell , he has an item that fives the spell.
now when i try to make a dummy cast a spell at the target of phoenix fire it doesn't work!

A unit starts casting an abiity

Ability equal to scatter bomb

Make dummy cast ...... at position: Target of abiity being cast.