Is it permissible to design a marketing video for an institution that buys loans from banks and reduces the interest rate and monthly installment for those in default?

As long as this institution is based on usurious loans and the purchase of debts, its work is forbidden. Therefore, it is not permissible to work in promoting and marketing for it, because doing so constitutes aiding in sin and aggression. The one who aids and causes something is a participant with the doer in the consequence of their action. Al-Maziri said: "Assisting in what is not permissible is not permissible, and Allah Almighty said: {And do not cooperate in sin and aggression} [Al-Ma'idah: 2]. And indeed, the one who points to good is like its doer, and similarly, the one who points to evil is like its doer." The rule is that means have the rulings of their ends.