Is swearing by "No, by Allah, I don't have it" when asked for money—while possessing less than the requested amount—considered a frivolous oath, or does it necessitate expiation by fasting for three days?

Lying is forbidden (haram), and its prohibition becomes more severe if a person swears an oath upon it. It is not permissible except out of necessity. To avoid lying, one can use ma'areed (allusions) and tawriya (equivocation), which is when the speaker utters words that bear two meanings: one near and unintended, and the other distant and intended.

If the questioner, by saying "By Allah, I don't have it," intended tawriya, then there is no sin upon him nor expiation.

However, if he did not intend tawriya and swore a false oath, then it is a yameen ghamus (a deeply sinful oath) that plunges its maker into sin or Hellfire. Its expiation is sincere repentance (tawba nasuh), a firm resolve not to repeat it, and an abundance of supererogatory acts of worship and good deeds. Some scholars have also stated that an expiation for an oath (kaffarat yameen) is obligatory, which is: feeding ten poor persons, or clothing them, or emancipating a slave. If one is unable to do so, then fasting for three days.