protected function _basketDifference(\OxidEsales\Eshop\Application\Model\Basket $oBasket)
    {
        $aDiff = $this->_getReservedItems();
        // refreshing history
        foreach ($oBasket->getContents() as $oItem) {
            $sProdId = $oItem->getProductId();
            if (!isset($aDiff[$sProdId])) {
                $aDiff[$sProdId] = -$oItem->getAmount();
            } else {
                $aDiff[$sProdId] -= $oItem->getAmount();
            }
        }

        return $aDiff;
    }