We have a workflow that locks the purchase invoice for all users except admins. We want to the other non-admin users to be able to edit that one invoice. What is the best way to achieve this? I believe I can't cancel the workflow on the invoice because we still want it to run against it after the non-admin roles edited that one purchase invoice. If we suspend the workflow, the new purchase invoices will not enter the workflow.

You could add a condition to your workflow lock action that only locks the invoice if its not "PO1234". Then remove it when done.

@Luis you would have to alter your workflow so that it no longer locks the invoice