Magento – Product stock qty management

I've developed a module to allow some minor changes to products from the frontend.

It works but the quantity management because the qty value is added and not replaced in cataloginventory_stock_item table.

I enabled query logging and in fact the sql query is:

UPDATE `cataloginventory_stock_item` SET `product_id` = ?, `stock_id` = ?, `qty` = qty+1488,  WHERE (item_id='5')

as you can see the qty value is added to the new qty value.

How to just update the value without adding it?

thanks

UPDATE

Here is the function to save the product:

protected function _initProductSave()
    {
        $product    = $this->_initProduct();
        $productData = $this->getRequest()->getPost('product');
        $dealData = $this->getRequest()->getPost('deal');

        /** setup default required values */

        $productData['visibility'] = 4;
        $productData['status'] = $dealData['deal_status'];
        $productData['is_deal'] = 1;
        //$productData['deal_type'] = $dealData['deal_type'];
        $productData['deal_from_date'] = $dealData['date_from'];
        $productData['deal_to_date'] = $dealData['date_to'];
        $productData['deal_target'];
        $productData['merchant_id'] = Mage::getModel('dailydeals/merchant')->getSessionMerchantId();


         /*** Inventory  */


            $productData['stock_data']['use_config_manage_stock'] = 1;


        /*** Websites   */
        if (!isset($productData['website_ids'])) {
            $productData['website_ids'] = array();
        }

        if(empty($productData['website_ids'])){
            $websites = Mage::app()->getWebsites();
            foreach($websites as $_website){
                $productData['website_ids'][] = $_website->getId();
            }
        }   

        /*** Media      */      
        $wasLockedMedia = false;
        if ($product->isLockedAttribute('media')) {
            $product->unlockAttribute('media');
            $wasLockedMedia = true;
        }

        $product->addData($productData);

        if ($wasLockedMedia) {
            $product->lockAttribute('media');
        }

        if (Mage::app()->isSingleStoreMode()) {
            $product->setWebsiteIds(array(Mage::app()->getStore(true)->getWebsite()->getId()));
        }

        /*** Check "Use Default Value" checkboxes values       */
        if ($useDefaults = $this->getRequest()->getPost('use_default')) {
            foreach ($useDefaults as $attributeCode) {
                $product->setData($attributeCode, false);
            }
        }

        /**
         * Initialize product categories - added in v0.1.6
         */
        $categoryIds = $this->getRequest()->getPost('deal_categories');
        if (null !== $categoryIds) {
            if (empty($categoryIds)) {
                $categoryIds = array();
            }
            $product->setCategoryIds($categoryIds);
        }


        /**
         * Initialize data for configurable product
         */
        if (($data = $this->getRequest()->getPost('configurable_products_data'))
            && !$product->getConfigurableReadonly()
        ) {
            $product->setConfigurableProductsData(Mage::helper('core')->jsonDecode($data));
        }
        if (($data = $this->getRequest()->getPost('configurable_attributes_data'))
            && !$product->getConfigurableReadonly()
        ) {
            $product->setConfigurableAttributesData(Mage::helper('core')->jsonDecode($data));
        }

        $product->setCanSaveConfigurableAttributes(
            (bool) $this->getRequest()->getPost('affect_configurable_product_attributes')
                && !$product->getConfigurableReadonly()
        );

        /**
         * Initialize product options
         */
        if (isset($productData['options']) && !$product->getOptionsReadonly()) {
            $product->setProductOptions($productData['options']);
        }

        $product->setCanSaveCustomOptions(
            (bool)$this->getRequest()->getPost('affect_product_custom_options')
            && !$product->getOptionsReadonly()
        );      

        Mage::dispatchEvent(
            'catalog_product_prepare_save',
            array('product' => $product, 'request' => $this->getRequest())
        );

        return $product;
    }