Follow these steps to modify a customer receipt in the Point of Sale:

1. Add the following code in the /static/src/js/pos_demo.js file. This will add extra data in the receipt environment:

var screens = require('point_of_sale.screens');

screens.ReceiptScreenWidget.include({
    get_receipt_render_env: function () {
        var receipt_env = this._super();
        var order = this.pos.get_order();
        var saved = 0;
        _.each(order.orderlines.models, function (line) {
            saved += ((line.product.list_price - line.get_base_price()) * line.quantity);
        });
        receipt_env['saved_amount'] = saved;
        return receipt_env;
    }
});

2. Add the following code in /static/src/xml/pos_demo.xml. This will extend the default receipt template and add our customization:

 <t t-extend="PosTicket">
 <t t-jquery=".receipt-change" t-operation="after">
 <t t-if="saved_amount">
 <br/>
 <div class="pos-center-align">
 <t t-esc="widget.format_currency(saved_amount)"/>
 you saved in this order
 </div>
 </t>
 </t>
</t>

Update the pos_demo module to apply the changes. After that, add a product with the discount and check the receipt; you will see one extra line in the receipt: