contract/contract_variable_quantity/views/contract_line_views.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2019 Tecnativa - Pedro M. Baeza
     License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl). -->
<odoo>

    <record id="contract_line_tree_view" model="ir.ui.view">
        <field name="name">contract.line tree view (in contract) - Add variable qty fields</field>
        <field name="model">contract.line</field>
        <field name="inherit_id" ref="contract.contract_line_tree_view"/>
        <field name="arch" type="xml">
            <field name="quantity" position="before">
                <field name="qty_type"/>
                <field
                    name="qty_formula_id"
                    attrs="{'invisible': [('qty_type', '!=', 'variable')]}"
                />
            </field>
            <field name="quantity" position="attributes">
                <attribute name="attrs">{'invisible': [('qty_type', '!=', 'fixed')]}</attribute>
            </field>
        </field>
    </record>

</odoo>