hire unit trigger, need help !!

i need a trigger, when a hero buy a hire (neutral unit from mercenary camp) then receive half of the price of unit after then buy...
but is only one hero can use this passive ability...

Use the Generic Unit Event: A unit sells a unit
And use the Integer Condition: Level of [The Passive Ability] for Buying Unit > 0

The Trigger should be like this:
Unit - A unit Sells a unit
(Level of (your passive skill) for (Buying unit)) Equal to 1
Player - Add (Point-value of (Sold unit)) to (Owner of (Buying unit)) Current gold

Set te custom value of every unit that can be sold to the half of the gold price.