When a stakeholder demands the implementation of requirements that are out of scope without additional cost, the project manager should refer to the project's statement of work (SOW). The SOW is a formal document that defines the work activities, deliverables, and timeline a vendor will execute for a customer. It serves as a legally binding agreement that specifies what is included in the project scope and, by extension, what is not. If the stakeholder's demands are not included in the SOW, they are considered out of scope, and the project manager should clarify this to the stakeholder. Implementing out-of-scope requirements without adjusting the project's cost, time, and resources can lead to scope creep, which can jeopardize the project's success.
References: The answer is verified with the principles found in the PMI's A Guide to the Project Management Body of Knowledge (PMBOKGuide) and the Professional in Business Analysis Reference Materials provided by PMI12345.