pos/pos_statement_closing_balance/views/pos_session_views.xml

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
    <record id="view_pos_session_form" model="ir.ui.view">
        <field name="name">pos.session.form.view</field>
        <field name="model">pos.session</field>
        <field name="inherit_id" ref="point_of_sale.view_pos_session_form"/>
        <field name="arch" type="xml">
            <button name="open_cashbox" position="after">
                <button name="button_update_statement_ending_balance"
                    class="oe_stat_button"
                    attrs="{'invisible':[('state', '=', 'closed')]}"
                    icon="fa-money"
                    type="object" context="{'balance': 'end'}">
                    <span class="o_stat_text">Update Ending Balances</span>
                </button>
            </button>
        </field>
    </record>
</odoo>