It's a little hard to tell what your argument is. You seem to be saying that some people negotiate agreements piecemeal rather than by laying all of their demands on the table at once, which is true, and that this necessarily disadvantages one of the participants in the agreement, which doesn't seem plausible. But I am at a loss to discover either a more plausible meaning in your comment or any way in which this discussion is relevant to the question of whether salary negotiation is zero-sum, negative-sum, or positive-sum.