What should a husband do who cannot leave his wife with her mother, who previously worked in the stock market and refuses to spend on her daughter, while he is unable to take his wife with him or leave her alone with their infant daughter?

Working in usurious banks and the money earned from them are forbidden, unless the worker is ignorant of the prohibition. Money earned from usury is forbidden only for the one who earned it, and is not forbidden for someone who acquired it through a permissible means. It is permissible for children to eat from their father's money earned from usury, as it is lawful for them and a burden upon their father. This is analogous to the Prophet's acceptance of gifts from Jews who dealt in usury, and to the story of Barirah and her charity which became a gift for the Prophet.