MrYupi

$query->registerRuntimeField("store",[
            "data_type" => "\Bitrix\Catalog\StoreProductTable",
            'reference' => [
                'this.ID' => 'ref.PRODUCT_ID'
            ],
        ]);

        $query->registerRuntimeField("DETAIL_AMOUNT",[
            "data_type" => "integer",
            "expression" => array("sum(%s)", "store.AMOUNT")
        ]);