contract/contract_sale/views/sale_order.xml

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

    <record id="view_order_form_form_view" model="ir.ui.view">
        <field name="name">sale.order.form.view</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <field name="partner_id" position="after">
                <field name="contract_id" domain="[('partner_id', '=', partner_id)]"/>
            </field>
        </field>
    </record>

</odoo>