Inquiry regarding the procedure for finished goods exporters to issue a Purchase Certificate

Our company is an exporter of finished goods. To claim customs duty drawbacks on the finished products, we requested the raw material suppliers to issue a Certificate of Raw Materials and Sub-materials for Export (Gi-nap-jeung) or a Certificate of Raw Materials and Sub-materials for Export (Bun-jeung), covering the supply period from 2022 to June 2025. However, the raw material supplier has requested that we issue a Purchase Certificate to them, stating that they can issue the Gi-nap-jeung based on this Purchase Certificate. I would like to know what the specific procedure is for our purchasing team to issue a Purchase Certificate to the raw material supplier.