Show discount including tax, calculate discount excluding tax

Question

I want to display the discount amount including tax on cart overview page (totals summary).

• Magento 2.1.8

Relevant settings:

• Catalog prices: Excluding tax
• Apply Customer Tax: After discount
• Apply Discount On Prices: Excluding tax

Current situation in total summary within cart:

I cannot set Apply Discount On Prices to Including Tax, because Magento then calculates wrong totals. So in short I need to calculate discount amounts excluding tax but displaying including tax.

Answer 1

Please check the warning message in the Magento Back office in Configuration -> Sales -> Tax -> Calculation Settings -> Apply Discount On Prices:

To apply the discount on prices including tax and apply the tax after discount, set Catalog Prices to “Including Tax”.

To do what you want, you need to customize the Magento Tax module

Answer 2

I tried to implement observer for this. Maybe it help to someone.

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="sales_quote_address_collect_totals_after">
        <observer name="vendor_module_fix_total_discount_amount" instance="Vendor\Module\Observer\FixTotalDiscountAmount" />
    </event>
</config>

and its implementation

<?php

namespace Vendor\Module\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Pricing\PriceCurrencyInterface;
use Magento\Quote\Model\Quote;
use Magento\Quote\Model\Quote\Address\Total;
use Magento\Tax\Model\Config as TaxConfig;

class FixTotalDiscountAmount implements ObserverInterface
{
    private TaxConfig $taxConfig;

    private PriceCurrencyInterface $priceCurrency;

    public function __construct(TaxConfig $taxConfig, PriceCurrencyInterface $priceCurrency)
    {
        $this->taxConfig = $taxConfig;
        $this->priceCurrency = $priceCurrency;
    }

    public function execute(Observer $observer)
    {
        /** @var Quote $quote */
        $quote = $observer->getData('quote');

        /** @var Total $total */
        $total = $observer->getData('total');

        if (!$this->taxConfig->displayCartPricesInclTax()) {
            // Don't need to add tax into discount.
            return;
        }
    
        if (
            $this->taxConfig->applyTaxAfterDiscount() // Apply Tax After Discount
            && !$this->taxConfig->discountTax() // Apply Discount on Prices Excluding Tax
            && !$this->taxConfig->priceIncludesTax() // Catalog Prices Excluding Tax
        ) {
            // Add "missing" Discount Tax from all Items
            foreach ($quote->getAllVisibleItems() as $item) {
                $discountTax = $item->getRowTotalInclTax() - $item->getRowTotal() - $item->getTaxAmount();
                $baseDiscountTax = $item->getBaseRowTotalInclTax() - $item->getBaseRowTotal() - $item->getBaseTaxAmount();

                $discountAmount = $total->getDiscountAmount() - $discountTax; // minus because discounts are negative numbers
                $baseDiscountAmount = $total->getBaseDiscountAmount() - $baseDiscountTax;

                $total->setDiscountAmount($this->priceCurrency->roundPrice($discountAmount));
                $total->setBaseDiscountAmount($this->priceCurrency->roundPrice($baseDiscountAmount));
            }
        }
    }
}