Unsold items that are returned, for example Christmas Cola, often come from different orders and are thus the subject of different invoices (one per order). Since it is not possible to refer to a single invoice, an invoicing period must be used in the credit note which covers the relevant invoices. Here is an example:
Credit returned unsold items (pdf) >
If the unsold items can be associated with a specific delivery/invoice then this can be referred to in the usual way in the credit note. (No example is given here for this alternative).