pos/pos_order_return/views/pos_order_view.xml

<?xml version="1.0"?>
<!-- Copyright 2016-2018 Sylvain LE GAL (https://twitter.com/legalsylvain)
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).-->

<odoo>

    <record id="action_pos_partial_return_wizard" model="ir.actions.act_window">
        <field name="name">Partial Return Wizard</field>
        <field name="type">ir.actions.act_window</field>
        <field name="res_model">pos.partial.return.wizard</field>
        <field name="view_type">form</field>
        <field name="view_mode">form</field>
        <field name="target">new</field>
    </record>

    <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>

    <record id="view_pos_order_line_form" model="ir.ui.view">
        <field name="model">pos.order.line</field>
        <field name="inherit_id" ref="point_of_sale.view_pos_order_line_form"/>
        <field name="arch" type="xml">
            <group position="after">
                <group col="4" string="Refund">
                    <field name="returned_line_id" colspan="4"/>
                    <field name="refund_line_ids" />
                </group>
            </group>
        </field>
    </record>

</odoo>