It is not possible to assign different currencies to different vendors. The purchase price in AGR is only mapped in one currency - the main accounting currency in your ERP. The purchase value in Orders is the order quantity * purchase price of the items.

See where purchase price is mapped from in the appropriate Set Up AGR article for your ERP

You can work around this issue by creating an order constraint with a minimum Purchase Value that is equivalent to the conversion of the minimum purchase value in different currencies to the same currency in which the purchase price is registered.