You can to add a discount to an invoice.
You can do this when creating the invoice or when editing the invoice.
To add the discount when creating an invoice you will go to the Finance Tab>Invoices.
Click on the green “+ New Invoice” button.
You will see the box to add a discount here:
To add a discount after the invoice has already been created, you will go to the Finance Tab>Invoices.
Find the invoice and click the pencil button to edit it.
You will then see the box to add the discount here: