I say, with Allah's guidance: Wiping over the khuffs becomes invalid by removing both khuffs or one of them; because the place of wiping is no longer covered. Removing them causes the state of impurity to affect the foot, and the khuff was what prevented it from spreading. If it is removed, it becomes invalid, even if only one khuff is removed; because if one of the khuffs is removed, it becomes obligatory to wash one of the feet, and thus it becomes obligatory to wash the other, as there is no combination of washing and wiping. The amount considered for the foot to exit the khuff is the majority of the foot reaching the leg of the khuff, as the majority is considered as the whole; this is to avoid the difficulty of a minor part of the foot exiting. Refer to: Sharh al-Wiqayah p.116, Radd al-Muhtar 1:183, and Allah knows best.