purchase_global_discount/purchase_view.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        
        <record id="purchase_order_line_form_global_discount_wizard_inherit1" model="ir.ui.view">
            <field name="name">purchase.order.line.form.global_discount.wizard.inherit1</field>
            <field name="model">purchase.order.line</field>
            <field name="inherit_id" ref="purchase.purchase_order_line_form" />
            <field name="arch" type="xml">
                <field name="price_unit" position="before">
                    <field name="orig_price_unit"/>
                </field>
                
                <field name="price_unit" position="after">
                    <field name="discount"/>
                </field>
            </field>
        </record>
        
        <record id="purchase_order_form_purchase_global_discount_wizard_inherit1" model="ir.ui.view">
            <field name="name">purchase.order.purchase.order.global_discount.wizard.inherit1</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form" />
            <field name="arch" type="xml">
                <field name="order_line" position="before">
                    <button name="%(action_purchase_global_discount_wizard_form)d" string="Set Global Discount" 
                            type="action" attrs="{'invisible':[('state','not in',['draft','sent'])]}" 
                            groups="purchase.group_purchase_user, purchase.group_purchase_manager" />
                </field>
                
                <xpath expr="//page[@string='Products']/field[@name='order_line']/tree" position="attributes">
                    <attribute name="editable"/>
                </xpath>
                <xpath expr="//page[@string='Products']/field[@name='order_line']/tree/field[@name='price_unit']" position="before">
                    <field name="orig_price_unit" />
                </xpath>
                
                <xpath expr="//page[@string='Products']/field[@name='order_line']/tree/field[@name='price_unit']" position="after">
                    <field name="discount" />
                </xpath>
                
                <xpath expr="//page[@string='Products']/field[@name='order_line']/tree/field[@name='company_id']" position="replace">
                </xpath>
            </field>
        </record>
        
    </data>
</openerp>