Is it permissible to pay zakat on the income from money—which is the family's sole source of livelihood—by analogy with the zakat on real estate?

Zakat is obligatory on wealth if it reaches the nisab (threshold) and a full year has passed on it, even if it is set aside for a need. Zakat includes the principal of the wealth and its lawful profits. As for profits from an interest-based (usurious) bank, they are haram (unlawful) and must be disposed of; Zakat is not obligatory on them. Zakat is obligatory only on the principal of the wealth, and a Muslim is required to repent and cease dealing with usurious transactions. Zakat is a cause for blessing and increase in wealth. It is not permissible to equate invested wealth with real estate to waive Zakat on it.