pos/pos_return_order/views/pos_order_view.xml

<?xml version="1.0"?>
<!--
Copyright (C) 2016-Today: La Louve (<http://www.lalouve.net/>)
@author: Sylvain LE GAL (https://twitter.com/legalsylvain)
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">
            <button name="refund" position="attributes">
                <attribute name="attrs">{'invisible':['|', ('state','=','draft'), ('returned_order_id', '!=', False)]}</attribute>
            </button>
            <button name="refund" position="after">
                <button name="%(action_pos_partial_return_wizard)d" string="Partial Return" type="action"
                    attrs="{'invisible':['|', ('state','=','draft'), ('returned_order_id', '!=', False)]}"/>
            </button>
            <notebook>
                <page string="Refund" attrs="{'invisible':[('returned_order_id', '=', False), ('refund_order_qty', '=', 0)]}">
                    <field name="returned_order_id" attrs="{'invisible':[('returned_order_id', '=', False)]}"/>
                    <field name="refund_order_ids" attrs="{'invisible':[('refund_order_qty', '=', 0)]}"/>
                    <field name="refund_order_qty" invisible="1"/>
                </page>
            </notebook>
            <xpath expr="//field[@name='lines']/form/group" position="after">
                <group col="4" string="Refund">
                    <field name="returned_line_id" colspan="4"/>
                    <field name="refund_line_ids" />
                </group>
            </xpath>
        </field>
    </record>

</odoo>