If you've accidentally put an invoice or credit on the wrong job, open up that job and click the Supplier Documents tab under the job phase the document is assigned to:

Click the ⋮ options button beside the document and choose Unassign from job phase:

This will return the invoice or credit to the Supplier Document Processing screen.

Make sure you don't void the document; this will irrevocably void the document in Fergus and your linked accounting package.