It is a bug in file Core/Business/Stock/Core_Business_Stock_StockManager.php at line 126
you should have like this:
$stockAvailable->quantity = $stockAvailable->quantity + $delta_quantity;
$stockAvailable->id_product = (int)$product->id;
$stockAvailable->id_product_attribute = (int)$id_product_attribute;
$stockAvailable->update();