Does the money the father took from the man count as a debt that must be repaid to his sons, knowing that the man exploited the land for many years without rent, and are we sinful for not repaying the money before the man's death, and does repaying the debt to the man's sons absolve the father's conscience, or does his debt remain until the Day of Judgment?

Dispute and litigation cases must be presented to the Sharia courts for adjudication. The sale between the father and the man is void if the price was not agreed upon at the time of the contract, as it is a condition for the validity of a sale that the price be known and specified at the time of the contract. Otherwise, the sale is valid if the price was agreed upon, and the land belongs to the man's heirs, and you may demand the remaining price.

As for the man's utilization of the land after the father's death, if the sale was void and the ownership of the land transferred to you, then either you were donating it to him, or you may demand compensation for his utilization of the land, especially if silence is not considered permission. If your right against him is proven, it can be offset against what remained of the land's price owed by the father, with the obligation to pay the remaining price to the man's heirs to clear the deceased's liability.