You'll want to apply credit notes in your accounting package as this will then sync through to Fergus.

First you'll will need to reissue your customer invoice. You can refer to our article on Duplicating Invoices to assist with voiding the existing invoice and creating a copy.

Following this you can then add a new line item with a negative value.

XXX Missing screenshot of invoice that includes line item with negative sell XXX

You can call this item whatever you want.

Please note you can't create an invoice with a negative total.