If the customer has already been fully invoiced for the items on the job but they return some of them, what is the process?
Open the A/R invoice and copy through to an A/R credit memo. Now edit the line(s) so that the credit memo is for the required amount. Then add the document - creating the credit memo from the invoice uses the job costing links rather than having to map them manually.
Now go the Invoicing tab of the job and the jobline(s) will now have a status of partially invoiced.
For lines that have been partially delivered/invoiced, these will need to be split in order to have the quantity that you want to cancel or write off on a separate line.