Proactive dialogue systems, which are designed to guide conversations toward predetermined goals. However, contemporary LLMs predominantly function as passive assistants, mechanically executing human instructions. A key challenge contributing to this limitation is the inherent difficulty in acquiring and annotating high-quality training data for proactive dialogue. Consequently, the scarcity of such data results in a notable deficiency in the proactive conversational capabilities of current LLMs.In this paper, we introduce PANDA (Proactive Agent-based Negotiation Dialogue Augmentation), a method designed to generate accurate, complex, and diverse proactive dialogue data for a challenging task—financial dispute mediation—where a LLM acts as the mediator. PANDA leverages a novel self-evolving synthesis process to manage a pool of user profiles and generate dialogues through structured interactions between multiple LLM-driven agents. To ensure data fidelity, we propose a comprehensive evaluation framework and build a two-level validation system combining automated and expert human verification. Our experiments demonstrate that an 8B-parameter model, trained on our synthesized dataset, achieves state-of-the-art results in the task's evaluation framework. Its performance rivals top closed-source models guided by heavily engineered prompts, even when provided with only essential information.