Is living off the profits of money deposited in banks permissible (halal) or forbidden (haram) for a widow who has no other source of income, bearing in mind that she pays the Zakat on the money annually and keeps the principal amount to prepare her daughters for marriage?

If the profits and interests are generated from a permissible investment in an Islamic bank through a Mudarabah (definition) contract, then there is no objection to benefiting from them. However, if the money is in a usurious (riba-based) bank, it must be withdrawn and deposited in an Islamic bank, or safeguarded through a permissible means. If only a usurious bank is available, then it should be placed in a current account. If only a savings account is available, then the interest must be disposed of by spending it on the interests of Muslims, the poor, and the needy, and it is not permissible to benefit from it except in cases of poverty.

If one has used forbidden interest while ignorant of its prohibition, then repentance is required, and there is no obligation to give an equivalent amount in charity, based on the statement of Allah Almighty: ﴿So whoever receives an admonition from his Lord and desists, he may have what has already passed, and his affair is left to Allah﴾ [Al-Baqarah: 275]. However, if one was aware of its prohibition, then they must give an equivalent amount in charity. It is preferable to give it in charity even if one was ignorant of its prohibition, as a precaution and to clear one's conscience.

One must not leave the interest to the usurious bank, as this strengthens it.