What is the ruling on working in an engineering office that participates in studies for projects financed by usurious loans, and in the design and follow-up of these projects after the loan is obtained?

Cooperating with transgressors in their transgression and falsehood is prohibited, as Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." And because the Prophet, peace and blessings be upon him, "cursed the one who consumes Riba, the one who gives it, the one who writes it, and its two witnesses, and he said: 'They are all equal.'" Therefore, writing the contract between those involved in Riba and bearing witness to it is unlawful, as it involves assisting in falsehood.

If the first stage of your work aims to obtain an interest-based loan, it is not permissible for you to work in it, as that involves assisting and facilitating the path to it.

As for what comes after obtaining the loan, there is no prohibition in dealing with its holder, because its unlawfulness is attached to his responsibility, and dealing with him at that time would not be assisting him, since he has already taken it. After being received, the loan enters the possession of the borrower and becomes a debt upon him, however, in an interest-based loan, the borrower sins for dealing with Riba.