Required Documents for Issuing Purchase Confirmations and Zero-Rate Tax Application when Purchasing Subcontracted Services (SW, etc.)

The transaction structure is A (Overseas Client), B (Our Company, Domestic Prime Contractor), and C (Domestic Subcontractor, Outsourced Services and SW Supplier). Company C received a subcontract from us (B) and provided overseas services to A (Overseas Company). We are now attempting to pay the subcontracting fee to Company C, but when we requested a zero-rate tax invoice, Company C requested a Purchase Confirmation. I understand that a foreign currency-denominated sales contract is required to issue a Purchase Confirmation, but the contract between B and C was conducted solely using a KRW-denominated purchase contract. Please confirm if a Purchase Confirmation can be issued with the current document, or if not, what other documents are required.