purchase_discount/views/purchase_discount_view.xml

<openerp>
<data>
    <record model="ir.ui.view" id="purchase_discount_order_line_form">
        <field name="name">purchase_discount.order.line.form</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="after">
                <field name="discount"/>
            </field>
        </field>
    </record>
    <record model="ir.ui.view" id="purchase_discount_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"/>
            </field>
        </field>
    </record>
    
     <record model="ir.ui.view" id="purchase_discount_order_form">
         <field name="name">purchase.discount.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"/>
                 </xpath>
         </field>
     </record>
</data>
</openerp>