Update

function AddOrderProperty($code, $value, $order) {
   if (CModule::IncludeModule('sale')) {
      if ($arOrderProps = CSaleOrderProps::GetList(array(), array('CODE' => $code))->Fetch()) {
         $db_vals = CSaleOrderPropsValue::GetList(array(), array('ORDER_ID' => $order, 'ORDER_PROPS_ID' => $arOrderProps['ID']));
         if ($arVals = $db_vals->Fetch()) {
            return CSaleOrderPropsValue::Update($arVals['ID'], array(
               'NAME' => $arVals['NAME'],
               'CODE' => $arVals['CODE'],
               'ORDER_PROPS_ID' => $arVals['ORDER_PROPS_ID'],
               'ORDER_ID' => $arVals['ORDER_ID'],
               'VALUE' => $value,
            ));
         } else {
            return CSaleOrderPropsValue::Add(array(
               'NAME' => $arOrderProps['NAME'],
               'CODE' => $arOrderProps['CODE'],
               'ORDER_PROPS_ID' => $arOrderProps['ID'],
               'ORDER_ID' => $order,
               'VALUE' => $value,
            ));
         }
      }
   }
}