PMP Practice: Verify objective(s) of the project agreement is met

Birch Reyes is managing a six-month software implementation project for Wasabi Satellite Co, a data analytics firm. During contract negotiations with CloudStream Solutions, the preferred vendor for cloud infrastructure services, the vendor insists on a payment schedule requiring 60% upfront and 40% upon final delivery. Birch's organization typically uses a milestone-based payment structure (30% at contract signing, 40% at midpoint completion, 30% at final acceptance) to manage cash flow and ensure vendor accountability throughout the project. The vendor argues that their pricing is already competitive and they need the upfront payment to allocate dedicated resources. Birch has confirmed that CloudStream's technical capabilities are the best fit for the project requirements, and alternative vendors would require significant technical compromises. The project sponsor has delegated negotiation authority to Birch but expects her to secure favorable terms. What should Birch do FIRST?