Is it permissible for the deceased's wife to sell a piece of land to pay off a usurious debt?

If selling the land leads to the elimination or reduction of usurious gains, then the heirs are obligated to do so, as it benefits the deceased. This is because Allah records a servant's deeds and what results from them, as Allah Almighty says: "And give up what remains of riba, if you are believers" and "Indeed, We revive the dead and record what they have put forth and their traces." And the Prophet (peace be upon him) said: "Whoever introduces a good practice in Islam will have the reward for it and the reward of those who act upon it after him... And whoever introduces a bad practice in Islam will have the burden of it and the burden of those who act upon it after him." These usurious gains are a result of his actions.