spell doesn't aggro unit

Ok, right now you might be puzzled at the title to this thread.
Let me explain...
Normally when you cast any spell onto an unit that unit will start attacking the unit that casted the spell on him. I want to make a spell that when you cast it on an unit that unit doesn't attack the unit who casted the spell on him.

How can I go about doing this?

When the spell is cast search for nearby enemies of the target. If any are found (other than the caster), order that unit to attack them. Otherwhise, order it to move out of range or simply stop (I'm not sure what your ability is so I can't be more specific).

it is a spell that can be casted at anytime without the spell causing aggro which will increase the damage that the caster does to the unit with the buff.

Try a positive buff like inner fire as a dummy.

If it does damage just make it do 0 damage and trigger the damage from a source other than the hero.

Only problem then might be if the attacked unit tries to reach the other source.

I think it is down to the AI. What are the properties of the player?

what do you mean by properties of the player?

Is it a computer, creeps, etc., that owns the unit?

Only normally creeps/computer owned spawns don't attack whatever casts a spell on them, as far as I can remember (then again, I rarely use single target spells on units, but I'm pretty sure).

its a player owned unit casting spell onto a computer owned unit

If its just a few spells, I'd trigger the effects after basing it on a spell that wouldn't set off the aggro. If it is all, then change the AI.

You can try shockwave and make it unit targeted(Should work) and set wave distance to 0, and casting range to the value you want, that wont aggro the unit nor is it going to do something with it.(Impale works the same way)

Still will be able to target the ground.

Use something like inner fire/rejuvenation/lightning shield/etc.