I say, and with God's success: The wiping over the socks is invalidated by removing both socks or one of them; due to the departure of the place of wiping from its location. The removal causes the event to spread to the foot, and the sock was the barrier preventing its spread. Therefore, if one removes it, it becomes invalid, even if only one sock is removed; because if one removes one of the two socks, he must wash one of the feet, and thus he must wash the other, as there cannot be a combination of washing and wiping. The considered amount for the foot to exit the sock is the exit of most of the foot to the leg of the sock; because the majority has the ruling of the whole; and this is to avoid the hardship of the exit of the lesser part of the foot. Refer to: Sharh Al-Wiqayah p. 116, and Radd Al-Muhtar 1: 183, and God knows best.