Is it permissible to invest the money of the heirs of a deceased person, knowing that the source of the deceased's wealth is unknown to the heirs and to the investor?

The fundamental principle regarding Muslims is their integrity, and regarding their wealth, it is lawful. This principle is not to be deviated from except with conclusive evidence or a predominant assumption. Therefore, there is no harm in dealing with wealth of unknown origin through partnership, investment, inheritance, or other means.