What is the ruling on a programmer developing a program for loans and interest (riba) without prior knowledge of its prohibition, and must he complete the work on it given that the manager is aware of its prohibition and the programmer refuses?

You are not permitted to complete this program after becoming certain that it involves the management of usurious transactions, based on the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." Furthermore, it is incumbent upon your manager to repent to Allah Almighty for what he has done, by abandoning these actions, regretting them, and resolving not to return to them. One must seek lawful means of earning a living, lest they be among those who consume people's wealth unjustly, or bring forbidden money into their family; for the Prophet, peace and blessings be upon him, said to Ka'b ibn Ujrah: "Indeed, no flesh that has grown from unlawful earnings will enter Paradise; the Fire is more deserving of it."