Most negotiators have experienced being confronted with a negotiation that they approached based on a misleading pretext. You may come to an interaction with a fundamental pretext as to what it will entail. If it is not what was expected, you must recalibrate with the new focus. You cannot morph one deal into another on the fly confidently if there is any measure of complexity. This process tactic is the corollary to the walk-away, or Fisher and Ury’s Best Alternative to Negotiated Agreement. When the fundamental assumptions of the relationship turn out to be different than what was expected, a recalibration is in order.
For example, you may go into a negotiation with pricing, timing, and key performance indicators assuming a strategic relationship, only to find out the other party was commoditizing you as a spot buy.
Simply communicate that this is beyond what you thought and ask if there is any value in proceeding given that fundamental assumptions were made based on prior communications, which are not now the case.