All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Generally speaking, people are of the view that usurious money encompasses the capital and its interests. However, the capital belongs to its owner; therefore, it is liable for Zakah (obligatory charity), as indicated by the following verse: "But if ye turn back, ye shall have your capital sums: Deal not unjustly, and ye shall not be dealt with unjustly." {Al-Baqarah/279}.
In principle, interests shouldn`t be owned because they are unlawful money that should be given, in full, to charity. Moreover, no Zakah is due on such money because spending it on the poor and needy is an obligation. This is because Allah, The Almighty, accepts only that which is lawful. He says: "O ye who believe! Give of the good things which ye have (honourably) earned. {Al-Baqarah/267}.
In addition, the Messenger of Allah (PBUH) said, “Allah the Almighty is Good and accepts only that which is good." {Muslim}. In terms of Islamic Sharia, unlawful money doesn`t have a value because it originates from an unlawful source. And Allah knows best.
