Workflow lock PO record if related bill exists

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.