Hi,
Why your business want to enter discount details in MIRO screen as MIRO used to capture vendor invoice details for your purchase order.
If discount are there- you can keep during purchase order creation by keeping a discount condition in it in your MM pricing procedure.
OR
You can have allow discount during payment (F-53) to your vendor for your invoice amount for your purchase order.
OR
If any case- any price differs or vendor again send invoice( adjust of value only), You can create subsequent debit or subsequent credit (based on nature of value) in t.code MIRO after your invoice document posted for your purchase order.
Regards,
Biju K