pos/pos_ticket_salesman_firstname/static/src/xml/pos.xml

<?xml version="1.0" encoding="UTF-8"?>

<templates id="template" xml:space="preserve">

    <t t-extend="PosTicket">
        <t t-jquery="div[class='receipt-user']" t-operation="replace">
            <div class="receipt-user">
                User: 
                <t t-if="widget.pos.config.receipt_salesman_firstname" t-esc="widget.pos.get_cashier().firstname"/>
                <t t-else="" t-esc="widget.pos.get_cashier().name"/>
                <br />
            </div>
        </t>
    </t>

</templates>