checkout_cart_update_items_before

config.xml just events

<events>
	<catalog_product_get_final_price>
		<observers>
		  <magestore_productoffer_observer>
			<type>singleton</type>
			<class>productoffer/observer</class>
			<method>catalog_product_get_final_price</method>
		  </magestore_productoffer_observer>
	   </observers>			
	</catalog_product_get_final_price>
	<checkout_cart_update_items_before>
		<observers>
		  <magestore_productoffer_observer>
			<type>singleton</type>
			<class>productoffer/observer</class>
			<method>checkout_cart_update_items_before</method>
		  </magestore_productoffer_observer>
	   </observers>
	</checkout_cart_update_items_before>
	<catalog_controller_product_init_before>
		<observers>
			<magestore_productoffer_observer>
				<type>singleton</type>
					<class>productoffer/observer</class>
				<method>productinitbefore</method>
			</magestore_productoffer_observer>
		</observers>
	</catalog_controller_product_init_before>

	<checkout_type_onepage_save_order_after>
		<observers>
		  <magestore_productoffer_observer>
			<type>singleton</type>
			<class>productoffer/observer</class>
			<method>checkout_type_onepage_save_order_after</method>
		  </magestore_productoffer_observer>
	   </observers>			
	</checkout_type_onepage_save_order_after>
</events>

Observer.php

 <?php 

public function checkout_cart_update_items_before($observer)
	{
        $event = $observer->getEvent();
        $products = $event->getInfo();
        $customer = Mage::getSingleton('customer/session');
        $items = $observer->getCart()->getQuote();
        foreach($products as $key => $value){
            $product = $items->getItemById($key);
            if($customer->getId()){
                $product_id = $product->getProductId();
                $customer_id = $customer->getId();
                $collection = Mage::getModel('productoffer/productoffer')
                                ->getCollection()
                                ->addFieldToFilter('customer_id',$customer_id)
                                ->addFieldToFilter('product_id',$product_id)
                                ->addFieldToFilter('status',2)
                                ->getFirstItem()
                                ;
                if($collection->getId()){
                    $products[$key]['qty'] = 1;
                }
            }
        }
        return $this;
	}