contract/contract_sale/views/contract_line.xml

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

    <record id="account_analytic_invoice_line_view_tree"
            model="ir.ui.view">
        <field name="name">account.analytic.invoice.line.tree (in contract_sale)</field>
        <field name="model">account.analytic.invoice.line</field>
        <field name="inherit_id"
               ref="contract.account_analytic_invoice_line_view_tree"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='discount']" position="attributes">
                <attribute name="groups">sale.group_discount_per_so_line
                </attribute>
            </xpath>
        </field>
    </record>

</odoo>