Effective workplace negotiation requires balancing multiple objectives, including achieving task goals, preserving professional relationships, and resolving conflicts constructively. However, misunderstandings, misaligned preferences, and interpersonal friction often impede successful outcomes. Politeness mitigates these challenges by fostering trust, reducing tension, and preventing escalation, and persuasive communication helps overcome resistance, align preferences, and guide participants toward mutually beneficial agreements. Motivated by these insights, we present **DIPLOMAT**, a dialogue system for polite and persuasive workplace negotiation. To support its development, we introduce **PROWESS**, a dataset of multi-turn workplace negotiation dialogues generated via a multi-agent framework and enriched with negotiation strategies, politeness levels, and persuasive strategies. **DIPLOMAT** is trained using Dialogue-Span-Aware Direct Preference Optimization (DSA-DPO), a novel preference learning objective that identifies key dialogue spans for preference alignment. This enables **DIPLOMAT** to generate contextually coherent responses that employ intended negotiation strategies, maintain politeness, and incorporate effective persuasion strategies throughout interactions. Automatic and human evaluations on **PROWESS** confirm that **DIPLOMAT** consistently outperforms baselines in generating coherent, polite, and persuasive negotiation responses.