When your character wants to make a deal, they're going to use Negotiation. If Coercion is threatening someone, Deception is lying to someone, Charm is sweet-talking someone, and Leadership is telling someone what to do, Negotiation is convincing someone to do what you want by giving them something they want in return. The skill in Negotiation comes from getting as much as you can out of a deal while offering as little as possible to the person you negotiate with.