pos/pos_session_pay_invoice/wizard/cash_invoice_in.xml

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
    <act_window
            id="action_pos_invoice_in"
            name="Pay invoice"
            res_model="cash.invoice.in"
            src_model="pos.session"
            view_mode="form"
            target="new"
            key2="client_action_multi"
        />
</odoo>