Is money earned from a business project, where the customers (individuals or companies) deal with usurious banks, considered permissible (halal) or forbidden (haram)?

If your work is permissible according to Sharia, then the source of the buyer's money, whether an individual or an institution, does not harm you, and you are not required to inquire about the source of their money. This is supported by the Prophet's (peace be upon him) and his companions' dealings with the Jews in buying and selling, even though they knew that the Jews consumed usury (riba) and illicit gain (suht). Therefore, rely on Allah and start your business project after studying its feasibility. "Whoever gives up something for the sake of Allah, Allah will compensate him with something better."