It is straightforward to apply a credit to a Permanent placement that has already been invoiced.
Follow these steps:
Click Invoicing and then click Perm Placements.
Select Placement entry and enter the placement ref in the Based On Previous Placement field.
Change the Debit field to Credit and click Save.
This will create the credit. To generate the credit note:
Click Invoicing and then click Perm Placements.
Select Generate Credit Placements.