pos/pos_invoicing/views/view_account_invoice.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
Copyright (C) 2013 - Today: GRAP (http://www.grap.coop)
@author Julien WESTE
@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_account_invoice_form" model="ir.ui.view">
        <field name="model">account.invoice</field>
        <field name="inherit_id" ref="account.invoice_form"/>
        <field name="arch" type="xml">
            <field name="reference" position="after">
                <field name="pos_pending_payment"/>
            </field>
        </field>
    </record>

</odoo>