Hi 

Please try this code,

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

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

        <t t-name="Customer" t-inherit="point_of_sale.ActionpadWidget" t-inherit-mode="extension" owl="1">

        <xpath expr="//div//button" position="replace">

            <t t-if="">

            <button class="disabled-mode mode-button button set-partner">Customer</button>

       </t>

            <t t-else="">

                <button class="button set-partner disabled-mode" t-att-class="{'decentered': isLongName}"

                        t-on-click="() => this.trigger('click-partner')">

                    <t t-if="!env.isMobile"><i class="fa fa-user" role="img" aria-label="Customer" title="Customer" /></t>

                    <t t-if="props.partner">

                        <t t-esc="props.partner.name" />

                    </t>

                    <t t-else="">

                        Customer

                    </t>

                </button>

            </t>

        </xpath>

    </t>

</templates>

By using your condition in the t-if statement, here I disabled the customer button in the payment screen as same as we can disable the customer button by extending the PaymentScreen,

Regards