What is the ruling on someone who searched for a long time for a job in Islamic banks to support his family, then turned to usurious banks after performing Istikhara? Is it permissible for him to work there if he is successful?

It is not permissible to earn a livelihood from a forbidden act, for Allah is good and accepts only what is good. He has commanded the believers to eat of the good things. Working in usurious banks is forbidden, and the Prophet, peace and blessings of Allah be upon him, cursed the one who consumes usury, the one who gives it, its scribe, and its two witnesses. A Muslim should seek lawful sustenance, and whoever abandons something for the sake of Allah, Allah will compensate him with something better. Istikhara (seeking guidance from Allah) is only for permissible matters and is not prescribed for forbidden matters.