pos/pos_order_mgmt/views/view_pos_order.xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- Copyright 2018 GRAP - Sylvain LE GAL
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-->
<odoo>

    <record id="view_pos_order_form" model="ir.ui.view">
        <field name="model">pos.order</field>
        <field name="inherit_id" ref="point_of_sale.view_pos_pos_form"/>
        <field name="arch" type="xml">
            <field name="session_id" position="after">
                <field name="returned_order_id" attrs="{'invisible': [('returned_order_id', '=', False)]}" />
            </field>
            <xpath expr="//div[@name='button_box']" position="inside">
                <button name="action_view_refund_orders" type="object" icon="fa-undo" class="oe_stat_button" attrs="{'invisible': [('refund_order_qty', '=', 0)]}">
                    <field string="Refunds" name="refund_order_qty" widget="statinfo"/>
                </button>
            </xpath>
        </field>
    </record>

</odoo>