pos/pos_order_return/views/product_product_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="view_product_template_form" model="ir.ui.view">
        <field name="model">product.template</field>
        <field name="inherit_id" ref="point_of_sale.product_template_form_view"/>
        <field name="arch" type="xml">
            <field name="to_weight" position="after">
                <field name="pos_allow_negative_qty" />
            </field>
        </field>
    </record>

</odoo>