Linked to #PrestaShop/PrestaShop/issues/35984

Thank you for your report and your analysis on the PrestaShop issue I recopy here:

I understand the issue with a Paypal module payment, but, that not the role of a payment gateway to manage the availability of the stocks but to the core. Only the core of PrestaShop sould manage if it's OK to create an order or not in this condition during the "validateOrder" action of the PaymentModule class.
Nevertheless, all payment gateways (I don't speak about PayPal only) cannot manage a refund after valid payment if the order failed for any reason. I suppose, it kindly helpfull to create an order without enough stock instead of an orphelin payment without order on PrestaShop ? Perhaps that's the reason why the core doesn't stop teh order creation. Not sure of that.
At least, there always was a timelaps between the user payment on the "bank portal" and its return on the shop. But, in a regular way, an order is created with a "reserved stock" different of the "physical stock". If you deduce a stock manually in the backoffice you deduce directly from the physical stock you increase the probability of this issue.

Hi @clotairer, the problem is that checking ValidateOrder would be useless as the payment has already been made. Basically Prestashop, from version 8, when you click on the order confirmation button, checks the quantity of products in the cart, it does so by executing an ajax call to the endpoint stored in the "window.prestashop.urls.pages.order" variable and passing as parameter "action" => "checkCartStillOrderable", here is the code I am referring to:

https://github.com/PrestaShop/PrestaShop/blob/2937cae599dcf5b4ada6914925eccdb2fa6cbfc3/themes/_core/js/checkout-payment.js#L173

The PayPal module should do the same thing as the ps_checkout module which checks the quantities before confirming the payment. Here is the module code I am referring to: https://github.com/PrestaShopCorp/ps_checkout/blob/6bba634f4ccdd5d0a01f2879f3073f963f9040e8/controllers/front/validate.php#L52

This method, in case of invalid quantities, raises an exception that prints an error in the cart, https://github.com/PrestaShopCorp/ps_checkout/blob/6bba634f4ccdd5d0a01f2879f3073f963f9040e8/controllers/front/validate.php#L312C1-L315C27