contract/product_contract_variable_quantity/views/product_template.xml

<?xml version="1.0" encoding="utf-8"?>

<!--
    Copyright 2018 ACSONE SA/NV.
    License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-->

<odoo>

    <record id="product_template_form_view" model="ir.ui.view">
        <field name="name">account.invoice.select.contract (in
            product_contract_variable_quantity)
        </field>
        <field name="model">product.template</field>
        <field name="inherit_id" ref="product.product_template_form_view"/>
        <field name="arch" type="xml">
            <xpath expr="//page[@name='contract']"
                   position="inside">
                <group>
                    <field name="qty_type"
                           attrs="{'required': [('is_contract', '=', True)], 'invisible': [('is_contract', '!=', True)]}"/>
                    <field name="qty_formula_id"
                           attrs="{'required': [('qty_type', '=', 'variable')], 'invisible': [('qty_type', '!=', 'variable')]}"
                    />
                </group>
            </xpath>
        </field>
    </record>

</odoo>