Buying mana?

I want a trigger so when you type "buy mana" a building is given 100 mana and it subtracts 100 gold every time u type it but i cant see a way to do it. the bit im having trouble with is adding the mana. i no it would probably be ovbious but can someone please tell me. :(

thanks, i thought itd be simple like that

better idea:

edit a manapotion and make the item "use automatically when aquired = true"