Need assistance to configure EL Expression referring to PR fields

Summary:

Content (required):

While creating PR, if user doesn't select item code, he will able to select required UOM in PR 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?

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