Is it permissible to consider a debt that was owed by the deceased as charity on his behalf, or must his family be informed of it?

If you have a debt owed to you by your deceased friend, this debt has become a right for his heirs. You are not permitted to give it away as charity on his behalf; rather, you must return it to them, based on the Prophet's (peace be upon him) saying: "Whoever leaves behind wealth, it is for his heirs."