What is the ruling on the husband and mother-in-law continuing to deceive their neighbor about the success of their illusory project and paying her fictitious profits from the 8,0 pounds they took from her on the basis of a partnership, and are they required to inform her of the truth?

The actions of these two individuals regarding the mudarabah (profit-sharing partnership) funds in this manner constitute treachery and unlawful consumption of wealth. Their claim that the amount paid was a monthly profit is a deception. They must repent to Allah, return all the money to its owner, deducting what they had previously paid her. Delaying repayment when able to do so is forbidden, as the Prophet (peace be upon him) said: "The delay of a rich person (in paying debt) is oppression." They must also seek forgiveness from the owner for their unauthorized use of her money. There is no basis for their misconception that returning the money is usury, as the mudarabah ended with their transgression, and they are responsible for guaranteeing the capital. Al-Buhuti said: "Mudarabah is a trust and an agency, and if the agent transgresses, it becomes usurpation." Since there was no profit from the money because they used it for their own benefit, what is incumbent upon them is to return the principal, in compliance with the words of Allah Almighty: "Indeed, Allah commands you to render trusts to whom they are due."