Hi Haidi,
If you just create a down payment request then that will still allow to change the purchase order net price. If the down payment request has been cleared by finance department then critical fields in the PO can't be changed like material code, quantity, net price etc.,
If the business requires to make the net price in grey field then Copy "NBF" and Create a Custom Field Selection Key, Then Do the Necessary Changes to the Copy and Assign it to the PO Document/Documents .
SPRO -> Material Management -> Purchasing -> Purchase Order -> Define Screen Layout at Document Level
Regards,