PMP Practice: Interpret the source and stage of the conflict

Xiu-Ying Huang is managing a solar farm installation project for Astrid Lindqvist-Holm Partners across three rural counties. The engineering lead, Ishaan Goswami, insists on using premium-grade inverters that exceed specifications, citing long-term reliability. The procurement manager, Rowan Fitzgerald-Yamamoto, strongly opposes this choice because it will push the project 18% over budget and delay delivery by five weeks. Both stakeholders have sent escalating emails to Xiu-Ying and are now refusing to attend joint planning sessions. Xiu-Ying has verified that both the premium and standard inverters meet all contractual and safety requirements. What should Xiu-Ying do to manage this conflict?