I have my custom purchasing workflow in place and it works as expected. It also has a feature to allow for resubmission of approved POs and there are no issues on that front so far. However, I am recently asked to stop the users from modifying the PO if there is an associated bill for that PO.  I am trying to update the 'Lock Record' condition based on this but not sure what the condition should be. If someone can suggest this, it would be greatly appreciated. Thanks a lot in advance.

Look at the Status of the PO. Will have changed as soon as a Bill is applied.

Sorry, it didn't work in my case as I need to stop the resubmission if the bill exists, irrespective of its status. If it is created and saved, the PO status doesn't change 😞

if a bill is created for a PO, unless it's a partial bill, the PO status would change to Billed instead of pending billing. if you mean you also need to restrict it upon partial bills, you'll probably need to use like a saved search field or something to indicate on the PO if it has a bill created and then run your WF off that condition.

@Rookie in your workflow have the saved search as suggested by Amy. I would use a summary criteria counting the number of applying transactions.