What is the ruling on someone who swore an oath — "By God Almighty, I will pay you the amount" — in the context of a jest that turned into an argument? And is this oath considered binding, necessitating an expiation for breaking an oath (freeing a slave, feeding ten poor people, or fasting for three days), or is it a vain oath (laghw al-yameen)?

One should not joke excessively, and it is a condition that there be no lying in it, nor that it lead to hatred and estrangement. And whoever swears by Allah and then breaks his oath, an expiation for the oath is incumbent upon him: feeding ten poor people or clothing them or freeing a slave, and if he cannot find (any of these), then fasting for three days. What your friend did is not an idle oath, but rather a deliberate oath.