Need assistance to configure EL Expression referring to PO fields

Summary:

Need assistance to configure EL Expression referring to PO fields

Content (required):

While creating PO, if user doesn't select item code, he will able to select required UOM in PO line. If user selects Item, then UOM will default from item master and user should be restricted from making any changes to it. We have configured EL expression referring to a custom job role such that UOM field will be frozen for users having this role. But we need to put additional condition so that UOM field should be available for making changes if Item Number is blank. Any suggestions please?