oca-purchase/odoo-bringout-oca-purchase-workflow-purchase_discount/purchase_discount/views/purchase_discount_view.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <record model="ir.ui.view" id="purchase_order_line_form2">
        <field name="name">purchase_discount.order.line.form2</field>
        <field name="model">purchase.order.line</field>
        <field name="inherit_id" ref="purchase.purchase_order_line_form2" />
        <field name="arch" type="xml">
            <field name="price_unit" position="after">
                <field name="discount" optional="show" />
            </field>
        </field>
    </record>
    <record model="ir.ui.view" id="purchase_order_line_tree">
        <field name="name">purchase_discount.order.line.tree</field>
        <field name="model">purchase.order.line</field>
        <field name="inherit_id" ref="purchase.purchase_order_line_tree" />
        <field name="arch" type="xml">
            <field name="price_unit" position="after">
                <field name="discount" optional="show" />
            </field>
        </field>
    </record>
    <record model="ir.ui.view" id="purchase_order_form">
        <field name="name">purchase_discount.purchase.order.form</field>
        <field name="model">purchase.order</field>
        <field name="inherit_id" ref="purchase.purchase_order_form" />
        <field name="arch" type="xml">
            <xpath
                expr="//field[@name='order_line']/tree/field[@name='price_unit']"
                position="after"
            >
                <field
                    name="discount"
                    optional="show"
                    attrs="{'readonly': [('qty_invoiced', '!=', 0)]}"
                />
            </xpath>
            <xpath
                expr="//field[@name='order_line']/form//field[@name='price_unit']"
                position="after"
            >
                <field
                    name="discount"
                    optional="show"
                    attrs="{'readonly': [('qty_invoiced', '!=', 0)]}"
                />
            </xpath>
        </field>
    </record>
    <record model="ir.ui.view" id="purchase_history_tree">
        <field name="name">purchase_discount.history.tree</field>
        <field name="model">purchase.order.line</field>
        <field name="inherit_id" ref="purchase.purchase_history_tree" />
        <field name="arch" type="xml">
            <field name="price_unit" position="after">
                <field name="discount" />
            </field>
        </field>
    </record>
</odoo>