Is there a way to modify an approved purchase order? Once it is approved it seems to be locked and even an admin cannot modify it.
You probably have a workflow or a script locking the record once approved. Modify that workflow or script and you should be able to modify. To find it, got to Customization -> Scripted Records
That would work but it also kind of defeat the purpose of the approval. Could I restrict access to a certain role?
Admins can edit approved POs.