Is zakat obligatory upon a debtor or upon one who receives financial aid from his family, yet possesses the zakat threshold in the bank as a precaution for need?

Receiving financial aid does not preclude the obligation of Zakat for someone who possesses Zakat-eligible wealth reaching the nisab. As for disbursing Zakat to this individual, it is permissible if their debt consumes all their wealth, provided the debt was incurred for a permissible purpose and they are in need. If the debt does not consume all their wealth, it is permissible to disburse Zakat to them if the remaining wealth is insufficient for their needs.