Adding mana to a unit.

This might be a noob question but, I have an ability on a hero, and when you cast the ability it costs 100 mana, and then what i want the ability to do is to add 100 mana to a building that player owns. Is this possible? Plz help. I know it will be done through triggers, but i can't find a trigger that will add a certain amount of mana.

Change picked unit to whatever.

Thx.