What should I do about my father's insistence on creating a usurious investment certificate in my name and with my signature, and my refusal to continue with it? Is it permissible to deal with his money mixed with usury? What about the usurious funds in my accounts? Am I a partner in sin, and how can I purify myself from this?

It is impermissible to assist one's father in usurious transactions, and obedience to him in such matters is not obligatory. The ruling on these transactions must be gently explained to him. If the father's money is mixed and the illicit portion does not predominate, there is no harm in benefiting from what he spends or gives. However, if most or all of it is illicit, one should not benefit from it except out of necessity. Some scholars have held that it is permissible to benefit from a parent's money acquired through usurious transactions. According to this view, it is permissible to accept money given by the father that was acquired from usurious interest, and one is not obliged to dispose of it.