Well I haven't played Stronghold so I'm not sure I understand what you mean by raising and lowering taxes exactly. It might help us understand a bit if you post this trigger you already have, but in terms of abilities I would suggest you do something like this:
1) In the object editor abilities section click create new custom ability and choose the heroes list of base abilities and find channel (And name this ability lower tax I guess)
2) Change the Art Button Positions from 2, 0 to 0, 2. (To make the button be in the bottom left if that's where you want it)
3) Delete the Art Caster, Art Effect, and Art Target unless you want there to be some special effect when you click the button
4) Change the icon to maybe Gold Coins in the item icons section.
5) Set stats-levels to 1 and Stats- hero ability to false
6) Change the hotkey normal to whatever you want the hotkey to be
7) Change the Base Order ID to, I don't know, ChemicalRage or something. Just not channel. Change the follow through time to 0, change disable other abilities to false, and check mark visible under data options.
8) Change your tooltip to something like |c00ff0000(Hotkey) |c00ffff00Lower Taxes
9) change your extended tooltip to explain what lowering taxes does
Ok done there. Now copy/paste that ability and name the new one Raise Taxes. Change its button position to maybe 1,2. Change its base order ID to Cloud of Fog. Change the hotkey and the tooltips. And find a different icon.
So now you have your abilities.
Now you just need to make your trigger run when the abilities are cast, so make the event of one trigger "a unit starts the effect of an ability" and the condition of that trigger be "Ability being cast equal to Lower Taxes". Then have your actions.
Then make a second trigger for Raise Taxes. And you should be good. But unless you tell us more about what exactly you want, I can't be sure I answered your question.
