Is wudu' obligatory if one removes the leather sock?

The Shaykh : One wipes over the leather sock, but if he removes it, wudu' becomes obligatory for him, according to the words of the jurists (may Allah have mercy on them). This is because it is not a substitute for what is underneath it; rather, the leather sock is a substitute for what is underneath it. Therefore, when what is underneath it becomes visible, purity is required of him.