At the first stage of CIPS Procurement and Supply Cycle (Understand need and develop a high-level specification), procurement professional mainly negotiate with internal stakeholders. They have a duty toproportionately and constructively challenge specification if there's genuine doubt over the need or how the need is expressed. This is called demand management. Their first duty is to the organisation's treasury, not to functional managers.
Demandmanagement including: negotiation/challenge between procurement and internal stakeholders over the need/requirement/specification. Remember that in any process or product, the greatest opportunity for cost reduction is at the design stage.