purchase_order_back2draft/views/purchase_order_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

        <record id="view_purhcase_order_form_draft" model="ir.ui.view">
            <field name="name">purchase.order.form.draft</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
                <field name="state" position="before">
                    <button name="button_purchase_draft"
                            states="cancel"
                            string="Back to draft"
                            type="object"
                            groups="base.group_no_one"/>
                </field>
            </field>
        </record>

    </data>
</openerp>