Is giving Zakat from wealth that has not reached the Nisab considered Sadaqah or Zakat?

Money that has not reached the nisab (threshold) is not subject to the obligatory Zakat. Giving Zakat from it at that point is considered a voluntary charity (sadaqa tatawwu') and does not substitute for the obligatory Zakat if it becomes due later before the completion of the hawl (one lunar year). It is not permissible to expedite Zakat before possessing the nisab, according to scholarly consensus (ijma').