What is the ruling on working on a project funded by an interest-based loan, and is the income of those working on this project considered lawful?

The son must advise his father to abandon taking out a usurious loan and, if possible, seek the help of scholars and students of knowledge. If the father does not respond and takes out a usurious loan, then the sin of usury falls upon him.

As for dealing with money acquired through a corrupt contract, it is permissible to deal with it in lawful ways, such as partnership or collecting rent, because the unlawfulness of money acquired through a prohibited contract is tied to the financial liability of the acquirer, not to the money itself. For the unlawful does not transfer by the transfer of money through a lawful means.

However, the son’s assistance to his father in preparing the usurious loan file is forbidden, because it is aiding in usury. The Prophet, peace and blessings be upon him, cursed the devourer of usury, its payer, its scribe, and its two witnesses.