What is the ruling on money that a father obtained from selling plots of land whose purchase price he had not yet paid, and is this money considered unlawful for the children who benefited from it?

The woman did well by giving your father more time to pay for the land. We ask Allah to help him return the rights to their owners, because delaying them is an injustice. The Prophet, peace and blessings be upon him, said: "The procrastination of the rich is injustice." The rightful owner may complain and demand the imprisonment of the procrastinator until he pays her right, as the Prophet, peace and blessings be upon him, said: "Delay by a wealthy person makes his honor and punishment lawful." If the procrastinator does not intend to fulfill his obligations, the matter is graver, as the Prophet, peace and blessings be upon him, said: "Whoever takes people's money intending to repay it, Allah will repay on his behalf; and whoever takes it intending to destroy it, Allah will destroy him." This does not mean that the borrowed money is forbidden for your father, but you must advise him and remind him of standing before Allah on a Day when neither wealth nor children will avail, and on the Day of Reckoning for good and bad deeds, so that he may fulfill the rights of their owners before death.