magento , the qty of products goes into minus on backorder

I’m trying to put qty to 0 if qty < 0, but it's not working for me i all time have qty on minus

my code don’t working for me

public function salesOrderSaveAfter($event)
    {
 $order = $event->getOrder();
            $orderIncrementId = $order->getIncrementId();

            foreach ($order->getAllVisibleItems() as $item) {
                $oldQty = (int)$item->getProduct()->getStockItem()->getQty();
                $qtyOrdered = (int)$item->getQtyOrdered();

                $differenceQty = $oldQty - $qtyOrdered;
            }

            //increase the price by 25% AND check for fires twice
            if ($differenceQty < 0) {
                $product_model = Mage::getModel('catalog/product')->load($item->getProductId());
                $price = $product_model->getPrice();
                $newPrice = $price + (($price * 25) / 100);
                $product_model->setPrice($newPrice);
                if ($product_model->getStockItem()->getBackorders() == "2") {
                    $product_model->setStockData(array(
                            'use_config_manage_stock' => 1,
                            'manage_stock' => 1,
                            'is_in_stock' => 1,
                            //'qty' => 0 don't working too
                        )
                    );
                    $product_model->setQty(0);
                }

                $product_model->save();
            }
}

I need when the quantity is less 0, for example -3, set quantity to 0