Disable item field based on value of other item field (PO)

Hi all,  I am very very new at Netsuite and the SuiteScript functionality. I want to disable a item field in a line item of a purchase order bases on the valua of another item field value.

I have tried this is a workflow but that did not work.

To specify, when an purchase line item has been selected, based on the item selected, the department related to that item is filled in. When this is the case, I would like the department field in the line item to be disabled. When the department is not filled in from the item data, the field needs to be enabled and editable.

I hope that somebody can (and is willing) to help my as noob. I apologize already if I made any mistake with posting this question here…..

Thanx in advance!