contract/contract_variable_quantity/views/contract_line.xml

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

    <record id="contract_line_form_view" model="ir.ui.view">
        <field name="name">contract.line.tree (in contract_variable_quantity)</field>
        <field name="model">contract.line</field>
        <field name="inherit_id"
               ref="contract.contract_line_form_view"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='quantity']" position="before">
                <field name="qty_type"/>
            </xpath>
            <xpath expr="//field[@name='quantity']" position="after">
                <field name="qty_formula_id"
                       attrs="{'required': [('qty_type', '=', 'variable')], 'invisible': [('qty_type', '!=', 'variable')]}"
                />
            </xpath>
            <xpath expr="//field[@name='quantity']" position="attributes">
                <attribute name="attrs">{'required': [('qty_type', '=', 'fixed')], 'invisible': [('qty_type', '!=', 'fixed')]}</attribute>
            </xpath>
        </field>
    </record>
</odoo>