flectra/addons/sales_discount/views/sale_view.xml

<?xml version="1.0" encoding="utf-8"?>
<flectra>
    <!-- Sale -->
    <record model="ir.ui.view" id="inherit_view_order_form">
        <field name="name">sale.order.inherit.form</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="priority" eval="20"/>
        <field name="arch" type="xml">
            <xpath expr="//button[@name='action_view_invoice']" position="replace">
                <button name="action_view_invoice"
                    type="object"
                    class="oe_stat_button"
                    icon="fa-pencil-square-o"
                    context="{'default_discount_method': discount_method, 'default_discount_amount': discount_amount, 'default_discount': discount,'default_discount_per': discount_per, 'var_for_default_get': True}"
                    attrs="{'invisible': [('invoice_count', '=', 0)]}">
                    <field name="invoice_count" widget="statinfo" string="Invoices"/>
                </button>
            </xpath>
            <xpath expr="//group[@name='left_panel']" position="inside">
                <separator string="Discount"/>
                <field name="discount_method" attrs="{'readonly': [('state', '!=', 'draft')]}"/>
                <field name="discount_amount" attrs="{'invisible': [('discount_method', '=', 'per')], 'readonly': ['|', ('state', '!=', 'draft'), ('discount_method', '=', False)]}"/>
                <field name="discount_per" attrs="{'invisible': ['|', ('discount_method', '=', 'fixed'), ('discount_method', '=', False)], 'readonly': [('state', '!=', 'draft')]}"/>
                <button name="calculate_discount" type="object" class="oe_edit_only oe_link" string="(Update Discount)" attrs="{'invisible': [('discount_method', '=', False)]}" confirm="If you want to apply global discount other discount methods will be reset."/>
            </xpath>
            <xpath expr="//group[@name='sale_total']/field[@name='amount_untaxed']" position="before">
                <field name="gross_amount" widget="monetary" options="{'currency_field': 'currency_id'}"/>
                <field name="discount" string="Discount" widget="monetary" options="{'currency_field': 'currency_id'}"/>
            </xpath>
        </field>
    </record>
</flectra>