Procurement Business Function Conversion Rate Type

Summary: We have a requirement to use a specific rate type (created custom rate type: Budget) in the Procurement Business Function. This needs to be defaulted to ALL PO's created for the BU. However, there is a different rate type in the Common Options for Payables and Procurement. We need this rate type = Corporate to default all invoices. Unfortunately, the rate type in the procurement business function is over written by the common options for payables and procurement, therefore the default rate on the PO's needs manual intervention.

Appreciate any advice how we can achieve this. We tried editing the Purchase order creation page, but we are unable to default the correct rate type.