Does an expiation for a broken oath (kaffarat yameen) become obligatory for someone who said to his friend, "It seems you are upset, by Allah," and the friend denied it? If so, what is the expiation?

What was said to the friend falls under the category of "laghw al-yameen" (thoughtless oaths), for which there is no expiation. This is because he swore based on what was apparent or presumed. Allah, the Almighty, says: (Allah will not hold you accountable for thoughtlessness in your oaths, but He will hold you accountable for what your hearts have earned). "Laghw al-yameen" is that which passes over the tongue unintentionally, or an oath taken by a person who believes himself to be truthful, only for it to turn out otherwise.