If I say "You may have two slices of my pizza in exchange for one slice of yours", then we're square, because that's the bargain we struck. If I want to give you two slices or three or eight, and I only want the one of yours, it's still fair, because it's all I want. If we make a deal, we state we're exchanging the slices one to one and you take two, I take another. Or I take something else of yours that's worth that slice of pizza - to me.
It's hard with pizza.
Think of it in terms of making deals with the mob? It's kinda like that. And it's physically uncomfortable for me to leave things uneven - unless we come to an agreement that you owe me a favour.