Hi Josh,
Have you tried in the sales document?
In Action tab > Incoming Document > Remove Incoming Document button:
As mentioned in the link you shared, only if the document is posted you need to remove the connection in the incoming document record. Otherwise, you can do it directly in the non-posted document.
"You can remove file attachments from non-posted documents at any time by deleting the related incoming document record. If the document is posted, then you must first remove the connection from the incoming document record."
Hope it helps
