manufacture/mrp_production_unreserve/views/mrp_production_view.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2017 Eficent Business and IT Consulting Services S.L.
     License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl-3.0) -->
<odoo>

    <record id="mrp_production_form_view" model="ir.ui.view">
        <field name="name">mrp.production.form - mrp_production_unreserved</field>
        <field name="model">mrp.production</field>
        <field name="inherit_id"
               ref="mrp.mrp_production_form_view"/>
        <field name="arch" type="xml">
            <button name="action_assign" position="after">
                <field name="unreserve_visible" invisible="1"/>
                <button name="button_unreserve" type="object"
                        string="Unreserve"
                        attrs="{'invisible': [('unreserve_visible', '=', False)]}"/>
            </button>
        </field>
    </record>

</odoo>