Magento2.4 - stock quantity deducted twice in observer

Question

I'm using Magento 2.4.3-p1 and I'm trying to decrease stock quantity with custom observer while placing an order, but salable quantity is decreasing twice, my events and observer file are this:

etc/events.xml

<?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_order_place_after">
        <observer name="update_qty_with_saleable_qty" instance="Vendor\Module\Observer\AfterPlaceOrderUpdateQty" />
    </event>
</config>

Vendor\Module\Observer\AfterPlaceOrderUpdateQty

<?php

declare(strict_types=1);

namespace Vendor\Module\Observer;

use Magento\Catalog\Model\Product as Product;
use Magento\CatalogInventory\Model\Stock\StockItemRepository;
use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Exception\CouldNotSaveException;
use Magento\Framework\Exception\InputException;
use Magento\Framework\Exception\NoSuchEntityException;
use Magento\Framework\Validation\ValidationException;
use Magento\InventoryApi\Api\Data\SourceItemInterface;
use Magento\InventoryApi\Api\SourceItemsSaveInterface;

/**
 * After Place Order Update Stock Quantity and Salable Quantity
 */
class AfterPlaceOrderUpdateQty implements ObserverInterface
{

    /**
     * @var SourceItemInterface
     */
    protected SourceItemInterface $sourceItemInterface;
    /**
     * @var SourceItemsSaveInterface
     */
    protected SourceItemsSaveInterface $sourceItemsSave;
    /**
     * @var Product
     */
    protected Product $product;
    /**
     * @var StockItemRepository
     */
    protected StockItemRepository $stockRepository;

    /**
     * @param SourceItemsSaveInterface $sourceItemsSave
     * @param SourceItemInterface $sourceItemInterface
     * @param StockItemRepository $stockRepository
     * @param Product $product
     */
    public function __construct(
        SourceItemsSaveInterface $sourceItemsSave,
        SourceItemInterface $sourceItemInterface,
        StockItemRepository $stockRepository,
        Product $product
    ) {
        $this->sourceItemsSave = $sourceItemsSave;
        $this->sourceItemInterface = $sourceItemInterface;
        $this->product = $product;
        $this->stockRepository = $stockRepository;
    }

    /**
     * @param $sku
     * @param $qty
     * @return AfterPlaceOrderUpdateQty
     * @throws NoSuchEntityException
     * @throws CouldNotSaveException
     * @throws InputException
     * @throws ValidationException
     */
    public function setQtyToProduct($sku, $qty): AfterPlaceOrderUpdateQty
    {
        $product = $this->product->getIdBySku($sku);
        $productStock = $this->stockRepository->get($product);
        $productQty = $productStock->getQty();
        $totalQty = $productQty - $qty;
        $this->sourceItemInterface->setSku($sku);
        $this->sourceItemInterface->setQuantity($totalQty);
        $this->sourceItemInterface->setStatus(1);
        $this->sourceItemInterface->setSourceCode('default');
        $this->sourceItemsSave->execute([$this->sourceItemInterface]);
        return $this;
    }

    /**
     * @param Observer $observer
     * @return void
     * @throws NoSuchEntityException
     * @throws CouldNotSaveException
     * @throws InputException
     * @throws ValidationException
     */
    public function execute(Observer $observer)
    {
        $order = $observer->getEvent()->getOrder();
        foreach ($order->getAllVisibleItems() as $item) {
            $productSku = $item->getSku();
            $productQty = $item->getQtyOrdered();
            $this->setQtyToProduct($productSku, $productQty);
        }
    }
}

Answer 1

Go to Admin Panel's Configuration and disable Decrease Stock When Order Is Placed in the Stock Options

Stores->Configuration->Catalog->Inventory->Stock Options->Decrease Stock When Order Is Placed "NO"

Answer 2

you should not use sales_order_place_after event for quantity update, that event will fire on every save statements Ex. $order->save().

try with "checkout_submit_all_after" or "sales_order_save_after"

your events.xml file would be with global scope app/code/{{vendor}}/{{module}}/etc/events.xml.

Answer 3

Are you using Multi Source Inventory feature in magento? If yes, it would automatically reduce the saleable quantity of an item when the order is placed.

Magento uses reservations to calculate and keep track of the salable quantity of each product assigned to a stock. When a customer places an order, Magento checks whether the quantity requested for each item is available for sale. If yes, Magento creates a reservation as an inventory request for each item, thereby reducing the salable quantity available for purchase.

https://devdocs.magento.com/guides/v2.4/inventory/reservations.html