How to enter write off amount for an invoice?

How to enter write off amount for an invoice?

In Settings>Templates>Customize Template>Total section for invoice customization, there is an option under "Show Payment Details" that says "Write off Amount." I would like to use that section to write off a dollar amount on an invoice. However, I am at the "New invoice" screen for a project and do not see a field in the Total section where I can enter a write off amount. Instead, I see only a field where I can enter a discount percentage. I do not want to discount the invoice by a percentage; I want to reduce the invoice by a dollar amount. Is it possible to do this?