1) Create a sales.xml file on app/code/Vendor/module/etc

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Sales:etc/sales.xsd">
    <section name="quote">
        <group name="totals">
            <item name="promodiscount" instance="Vendor\Module\Model\Quote\Discount" sort_order="500"/>
        </group>
    </section>
</config>

</group>

</section>

</config>

2) Create file Discount.php on this path app\code\Vendor\Module\Model\Quote\Discount

namespace Vendor\Module\Model\Quote;

class Discount extends \Magento\Quote\Model\Quote\Address\Total\AbstractTotal
{

    public function collect(
        \Magento\Quote\Model\Quote $quote,
        \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment,
        \Magento\Quote\Model\Quote\Address\Total $total
    ) {
        parent::collect($quote, $shippingAssignment, $total);

            $customDiscount = -20;

            $total->addTotalAmount('customdiscount', $customDiscount);
            $total->addBaseTotalAmount('customdiscount', $customDiscount);
            $quote->setCustomDiscount($customDiscount);
        }
        return $this;
    }

    /**
     * Assign subtotal amount and label to address object
     *
     * @param \Magento\Quote\Model\Quote $quote
     * @param Address\Total $total
     * @return array
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     */
    public function fetch(\Magento\Quote\Model\Quote $quote, \Magento\Quote\Model\Quote\Address\Total $total)
    {	
    	$customDiscount = 20;
        return [
            'code' => 'Custom_Discount',
            'title' => $this->getLabel(),
            'value' => $customDiscount
        ];
    }

    /**
     * get label
     * @return string
     */
    public function getLabel()
    {
        return __('Custom Discount');
    }
}

namespace Vendor\Module\Model\Quote;

class Discount extends \Magento\Quote\Model\Quote\Address\Total\AbstractTotal

{

public function collect(

\Magento\Quote\Model\Quote $quote,

\Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment,

\Magento\Quote\Model\Quote\Address\Total $total

) {

parent::collect($quote, $shippingAssignment, $total);

$customDiscount = -20;

$total->addTotalAmount('customdiscount', $customDiscount);

$total->addBaseTotalAmount('customdiscount', $customDiscount);

$quote->setCustomDiscount($customDiscount);

}

return $this;

}

/**

* Assign subtotal amount and label to address object

* @param \Magento\Quote\Model\Quote $quote

* @param Address\Total $total

* @return array

* @SuppressWarnings(PHPMD.UnusedFormalParameter)

public function fetch(\Magento\Quote\Model\Quote $quote, \Magento\Quote\Model\Quote\Address\Total $total)

{

$customDiscount = 20;

return [

'code' => 'Custom_Discount',

'title' => $this->getLabel(),

'value' => $customDiscount

];

}

/**

* get label

* @return string

public function getLabel()

{

return __('Custom Discount');

}

3) Create file checkout_cart_index.xml on path app\code\Vendor\Module\view\frontend\layout

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
	<referenceBlock name="checkout.cart.totals">
	   <arguments>
	       <argument name="jsLayout" xsi:type="array">
	           <item name="components" xsi:type="array">
	               <item name="block-totals" xsi:type="array">
	                   <item name="children" xsi:type="array">
	                       <item name="customdiscount" xsi:type="array">
	                           <item name="component"  xsi:type="string">Vendor_Module/js/view/checkout/summary/custom-discount</item>
	                           <item name="sortOrder" xsi:type="string">30</item>
	                           <item name="config" xsi:type="array">
	                               <item name="customdiscount" xsi:type="string" translate="true">Custom Discount</item>
	                           </item>
	                       </item>
	                   </item>
	               </item>
	           </item>
	       </argument>
	   </arguments>
	</referenceBlock>
    </body>
</page>

<?xml version="1.0"?>

<body>

<item name="component" xsi:type="string">Vendor_Module/js/view/checkout/summary/custom-discount</item>

<item name="customdiscount" xsi:type="string" translate="true">Custom Discount</item>

</item>

</argument>

</arguments>

</referenceBlock>

</body>

</page>

4) Create file checkout_index_index.xml on path app\code\Vendor\Module\view\frontend\layout

<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceBlock name="checkout.root">
            <arguments>
                <argument name="jsLayout" xsi:type="array">
                    <item name="components" xsi:type="array">
                        <item name="checkout" xsi:type="array">
                            <item name="children" xsi:type="array">
                                <item name="sidebar" xsi:type="array">
                                    <item name="children" xsi:type="array">
                                        <item name="summary" xsi:type="array">
                                            <item name="children" xsi:type="array">
                                                <item name="totals" xsi:type="array">
                                                    <item name="children" xsi:type="array">
                                                       <item name="customdiscount" xsi:type="array">
                                                            <item name="component"  xsi:type="string">Vendor_Module/js/view/checkout/cart/totals/custom-discount</item>
                                                            <item name="sortOrder" xsi:type="string">20</item>
                                                            <item name="config" xsi:type="array">
                                                                 <item name="template" xsi:type="string">Vendor_Module/checkout/cart/totals/custom-discount</item>
                                                                <item name="title" xsi:type="string" translate="true">Custom Discount</item>
                                                            </item>
                                                        </item>
                                                    </item>
                                                </item>
                                                <item name="cart_items" xsi:type="array">
                                                    <item name="children" xsi:type="array">
                                                        <item name="details" xsi:type="array">
                                                            <item name="children" xsi:type="array">
                                                                <item name="subtotal" xsi:type="array">
                                                                    <item name="component" xsi:type="string">Magento_Tax/js/view/checkout/summary/item/details/subtotal</item>
                                                                </item>
                                                            </item>
                                                        </item>
                                                    </item>
                                                </item>
                                            </item>
                                        </item>
                                    </item>
                                </item>
                            </item>
                        </item>
                    </item>
                </argument>
            </arguments>
        </referenceBlock>
    </body>
</page>

<body>

<item name="component" xsi:type="string">Vendor_Module/js/view/checkout/cart/totals/custom-discount</item>

<item name="template" xsi:type="string">Vendor_Module/checkout/cart/totals/custom-discount</item>

<item name="title" xsi:type="string" translate="true">Custom Discount</item>

</item>

<item name="component" xsi:type="string">Magento_Tax/js/view/checkout/summary/item/details/subtotal</item>

</item>

</argument>

</arguments>

</referenceBlock>

</body>

</page>

5) Create file custom-discount.js on path app/code/Vendor/Module/view/frontend/web/js/view/checkout/cart/totals

define(
    [
        'Vendor_Module/js/view/checkout/summary/custom-discount'
    ],
    function (Component) {
        'use strict';

        return Component.extend({

            /**
             * @override
             */
            isDisplayed: function () {
                return true;
            }
        });
    }
);

define(

[

'Vendor_Module/js/view/checkout/summary/custom-discount'

function (Component) {

'use strict';

return Component.extend({

/**

* @override

isDisplayed: function () {

return true;

}

});

}

);

6) Create file custom-discount.html on path app/code/Vendor/Module/view/frontend/web/template/checkout/cart/totals

<!-- ko if: isDisplayedCustomdiscountTotal() -->
<tr class="totals customdiscount excl">
    <th class="mark" colspan="1" scope="row" data-bind="text: title"></th>
    <td class="amount">
        <span class="price" data-bind="text: getCustomdiscountTotal()"></span>
    </td>
</tr>
<!-- /ko -->

</td>

</tr>

7) Create file on this path custom-discount.js app/code/Vendor/Module/view/frontend/web/js/view/checkout/summary

define(
    [
       'jquery',
       'Magento_Checkout/js/view/summary/abstract-total',
       'Magento_Checkout/js/model/quote',
       'Magento_Checkout/js/model/totals',
       'Magento_Catalog/js/price-utils'
    ],
    function ($,Component,quote,totals,priceUtils) {
        "use strict";
        return Component.extend({
            defaults: {
                template: 'Vendor_Module/checkout/summary/custom-discount'
            },
            totals: quote.getTotals(),
            isDisplayedCustomdiscountTotal : function () {
                return true;
            },
            getCustomdiscountTotal : function () {
                var price = totals.getSegment('Custom_Discount').value;
                return this.getFormattedPrice(price);
            }
         });
    }
);

define(

[

'jquery',

'Magento_Checkout/js/view/summary/abstract-total',

'Magento_Checkout/js/model/quote',

'Magento_Checkout/js/model/totals',

'Magento_Catalog/js/price-utils'

function ($,Component,quote,totals,priceUtils) {

"use strict";

return Component.extend({

defaults: {

template: 'Vendor_Module/checkout/summary/custom-discount'

totals: quote.getTotals(),

isDisplayedCustomdiscountTotal : function () {

return true;

getCustomdiscountTotal : function () {

var price = totals.getSegment('Custom_Discount').value;

return this.getFormattedPrice(price);

}

});

}

);

8) Create file custom-discount.html on path app/code/Vendor/Module/view/frontend/web/template/checkout/summary

<!-- ko if: isDisplayedCustomdiscountTotal() -->
<tr class="totals coupon-discount excl">
   <th class="mark" colspan="1" scope="row" data-bind="text: customdiscount"></th>
   <td class="amount">
       <span class="price" data-bind="text: getCustomdiscountTotal(), attr: {'data-th': customdiscount}"></span>
   </td>
</tr>
<!-- /ko -->

</td>

</tr>

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