Why don't you use a different field selection for NB document type.
Normally NB document type has the field selection of NBF.
Check the NBF field selection is not used in any other document type.
Then make the net price field display in OLME-Purchase Order-Define Screen Layout at Document Level
Double click on NBF then
Then when you are going to create PO from PR, the price will pick from PR, and user can't able to change the price.
And for manual PO, if you create info record, then the price will fetch from info record and can't be change in PO creation time.
If you don't want to use NBF, then you can create your own field selection coping NBF, and assign it to your document type.