help with a spell

help with a spell

I'm making a spell that allows the hero to cast spells for 30 seconds without using mana. Only problem is with the trigger. It doesn't work right. Mana is still used up.

mana font
    Events
        Unit - A unit Begins casting an ability
    Conditions
    Actions
        If (All Conditions are True) then do (Then Actions) else do (Else Actions)
            If - Conditions
                (Ability being cast) Equal to Stone I 
            Then - Actions
                If (All Conditions are True) then do (Then Actions) else do (Else Actions)
                    If - Conditions
                        ((Casting unit) has buff Mana Font ) Equal to True
                    Then - Actions
                        Unit - Set mana of (Casting unit) to ((Mana of (Casting unit)) + 9.00)
                    Else - Actions
                        Do nothing
            Else - Actions
                Do nothing

Please help me. BTW the spell uses only 9 mana.
Thanks

Trigger fires before spell is casted. Change event.

can't you just set the mana cost to zero?

If this doesn't work [changing event], create 2 abilities, one with normal cost, one with 0 cost [mana]. When hero uses ability allowing him to free cast spells, change abilities.