In an agile project, when there is a disagreement about a major new requirement among key stakeholders, the project manager should facilitate a meeting with the stakeholders and product owner to clarify the new need.
This approach ensures that all parties have a clear understanding of the requirement and can work towards a common goal. It also helps to prevent potential issues that could arise from misunderstandings or miscommunications. References: PMBOKGuide - Sixth Edition, Section 13.2.2.1: Plan Stakeholder Engagement