What is the ruling on participating in a project where 70% of its funding comes from the questioner through Islamic financing, and 30% comes from another investor who funds it with an interest-based loan?

We understand from the question that you wish to establish a project in which you participate with 70%, and another investor participates with 30%, which he will borrow from a usurious bank. You are asking about the ruling on partnering with him. The answer is that it is permissible if his participation does not entice him to usurious borrowing. Similarly, if he has already borrowed, then partnering with him is permissible because the prohibition of usurious loans pertains to the borrower's liability, not to the specific item it was used for. However, if he has not repented from this action, it is preferable not to partner with him. A Muslim should choose a just and trustworthy partner for himself, as the Prophet, peace and blessings be upon him, said: "The hand of Allah is over the two partners as long as they do not betray each other." And Allah Almighty says: "And indeed, many of the partners oppress one another, except for those who believe and do righteous deeds—and few are they." Therefore, jurists disliked partnering with a disbeliever or someone who does not avoid dealing with unlawful things.