In order to return any item, click on the Return goods option in the invoice menu. 
Goods can be returned only for approved invoices. It's possible to return only some of the goods. In that case the original invoice can be returned again, until all goods are returned.
Enter the returned items information in the items table , then click on [SAVE] button to save the invoice. 
Certain fields are protected from changes for the return invoices:
- partner details
- TAX included
Link to the original document is visible at the bottom of the invoice Details/Edit page. Additionally, it's possible to select return status.