Taking money from his wife and investing it with his own money without writing it down

Question: A man invested his wife's money with his own money, and she was somewhat content with this. His wife used to demand that he write her a document regarding the property for the amount of her money, so that she would be assured that her money would not go to the heirs after his death. However, her husband used to tell her: "These funds—meaning his funds and hers—are for you and your children after me." He passed away before writing her anything for the amount of her money. Does he bear any responsibility for this? And what is the obligation of the heirs regarding this matter? May Allah reward you with good.

Answer: In the name of Allah, and all praise is due to Allah, and may Allah send blessings and peace upon the Messenger of Allah, and upon his family and his Companions, and upon those who follow his guidance. As for what follows: It is obligatory for the husband, if he has money belonging to his wife, to write that down and to clarify it in a fixed document so that it is delivered to her after his death. It is obligatory for him to clarify this while he is healthy so that his conscience is cleared. If he dies and does not clarify this, it is obligatory for the heirs to pay her right from the principal of the estate, just as they must do for other creditors if this is established by evidence, or if they allow her to take it and believe her, provided they are sane and accountable. It is not permissible for the husband or anyone else, if he has a debt owed to someone, to remain silent and neglect it, so that rights are lost. Indeed, this is a great danger and a great injustice from which one must beware. Therefore, it is obligatory for every person who has a right belonging to another—whether he is a husband or not, or a wife or otherwise—to clarify this and write the debt in a legal document at the court, or with a known scribe whose writing is accepted, so that the right is delivered to its owner if Allah decrees death before repayment.
It is obligatory for the heirs to give this woman her right if it is established with them. If it is not established, then they owe nothing, and Allah will compensate her for it.
Her husband did wrong and engaged in what is not permissible for him. She has the choice: if she permits it and forgives him, she will have her reward. If she does not forgive, she will take her right from him on the Day of Resurrection, and nothing will be lost for her.
So you, O sister in Allah, the questioner, if you forgive and clear the husband, may Allah reward you with good. If you do not forgive and the heirs do not give you your right, then the matter is with Allah, and the reckoning is between you and him before Allah (Glorified and Exalted is He) and Allah is the Helper.