How is ill-gotten wealth calculated in the father's share of his deceased son's estate, which includes four houses and a bank balance, knowing that one of these houses was purchased by the son through a usurious bank, and the method of purchasing the rest of the houses is unknown?

The deceased's estate, including houses and financial assets, is to be divided among the heirs after disposing of any unlawful usurious gains by spending them on the welfare of Muslims. The principal of the lawful money is then divided. If the source of the money is unknown, whether lawful or unlawful, it is considered lawful by the consensus of scholars. The heirs must seek forgiveness for the deceased and supplicate for him. The father's share is one-sixth of the inheritance, and the wife's share is one-eighth, with the remainder going to the son. Siblings have no share unless there is a will, and this is all after settling the deceased's debts and executing his will.