In this paper two formal dialectic systems are described, a persuasion protocol (PP0) and a negotiation protocol (NP0), together with a method for shifting from an instance of a persuasion dialogue to an instance of a negotiation dialogue. The rationale for this kind of shift is explored in the context of the fallacy of bargaining. Such a dialectical shift is proposed as a valuable way to enable the participants in an argumentative dialogue to proceed towards a practical settlement when they are otherwise unable to persuade each other.