The Credit Notes feature is available in the Sales Pack extension.
Credit notes document adjustments made to previously issued invoices, such as returns, post-sale discounts, or mistake corrections. A credit note is usually created from an existing invoice. A credit note with the same line items as the invoice completely negates the invoice.
A credit note can also be used to negate an invoice that is deemed uncollectible. It fits this purpose because it reduces both the revenue and tax components of the outstanding amount.
Documentation
See more about the Credit Notes feature in the documentation.