Is a voluntary fast valid if the fasting person drinks an hour after the dawnathan, knowing that? And is expiation obligatory upon him if it is not valid?

The fast is invalid because it must extend from the break of dawn until sunset, as Allah Almighty says: (So now have sexual relations with them and seek what Allah has ordained for you, and eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the night).

Therefore, there is no reward for anyone who fasted this day because it did not conform to the Sharia, but there is no sin upon them because a voluntary fast may be broken. Expiation (kaffarah) is only obligatory for someone who had sexual relations with their spouse during the daytime in Ramadan while both were obligated to fast. In this case, expiation is incumbent upon both of them: freeing a slave. If one cannot find a slave to free, then fasting for two consecutive months. If one is unable to do that, then feeding sixty poor people.

However, if the couple is not obligated to fast, such as travelers, there is no blame upon them, and they should make up that day. There is no expiation upon them, even if they were fasting and then had sexual relations while on a journey that permits breaking the fast.