You can use a negative amount on a product line when creating a sales invoice. This can be useful for creating partial credit notes or discounts.
Start by going to Sales > Invoices and clicking the blue New invoice button. Add the invoice date, customer, currency, and (if needed) a description. Click Create.
To create a negative amount for a product, add a minus to the product's Quantity or Amount.
To see the invoice, open the side panel and click Preview.
Now, the invoice is ready to be booked and sent. To book and send it, click the blue button at the top right corner or click the arrow next to the blue button and choose Book.
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Create, book and send sales invoices