Is there not doubt regarding the remaining duration for wiping over the khuffs, based on the principle that the condition does not exist, which contradicts the rule "certainty is not removed by doubt"; and does someone who is certain of their purity and doubts the occurrence of impurity, then wears their khuffs, mean that their wiping is invalid based on this reasoning?

Firstly: Doubt concerning the remaining period for wiping over the khuffayn (leather socks) prevents wiping, by scholarly consensus. This is because the original rule is to wash the feet, and wiping is a concession conditioned on the duration of its period. Scholars have differed on whether this issue is an exception to the rule "certainty is not removed by doubt," or if it aligns with it, given that the original rule is washing the feet, and this certainty should not be abandoned due to doubt concerning the condition of wiping.

Secondly: Regarding one who is certain of purity but doubts the occurrence of hadath (minor ritual impurity), scholars have differed. Imam Malik held that such a person must perform wudu (ablution), while the majority of scholars maintained that they remain in a state of purity. Both parties invoked the rule "certainty is not removed by doubt," but from different perspectives. Malik viewed prayer as an obligation of certainty, and one's responsibility is not absolved by doubt. The majority, however, considered purity to be certain and hadath to be doubtful. Accordingly, one who doubts the occurrence of hadath is not permitted to wipe over the khuffayn according to Imam Malik's view, but is permitted according to the majority's view.