Hello Moe, 

Please find code in comment. 

Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari

Hi,

Try like below code.

python:

from odoo import fields, models, api

class AccountInvoice(models.Model):
    _inherit = 'account.move'

    discount_amount = fields.Float('Discount Amount', digits='Product Price')
    amount_due = fields.Float(compute='_compute_amount_due', digits='Product Price', store=True)

    @api.depends('amount_total', 'discount_amount')
    def _compute_amount_due(self):
        for record in self:
            record.amount_due = record.amount_total - record.discount_amount

xml:

<?xml version="1.0" encoding="utf-8"?>
<odoo>
        <!-- Add the discount_amount field to the invoice form view -->
        <record id="account_invoice_form" model="ir.ui.view">
            <field name="name">account.invoice.form</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.view_invoice_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name='amount_total']" position="after">
                    <field name="discount_amount"/>
                </xpath>
            </field>
        </record>
</odoo>

Regards