contract/agreement_legal_sale/views/sale_order.xml

<!-- Copyright 2019 Open Source Integrators
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl). -->
<odoo>

    <!-- Agreement Form View -->
    <record id="sale_order_agreement_form_view" model="ir.ui.view">
        <field name="name">sale.order.agreement.form.view</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="agreement_sale.sale_order_agreement_form_view" />
        <field name="arch" type="xml">
            <field name="agreement_id" position="attributes">
                <attribute name="readonly" eval='1' />
                <attribute
                    name="attrs"
                >{'invisible': [('agreement_id', '=', False)]}</attribute>
            </field>
            <field name="partner_id" position="after">
                <field name="agreement_template_id" />
            </field>
        </field>
    </record>

</odoo>