What is the condition of a deceased father in his grave, and is he at peace, knowing that he had debts and there was an intention to pay them, but his uncle did not fulfill his promise to pay them?

The soul of a believer is tied to their debt until it is paid off. Your father's debt must be paid from the lands he left behind. If your uncle prevents their sale, the matter should be taken to court. Your effort to free your father from his debt is a form of filial piety towards him after his death, and Allah knows best his state in his grave. The Prophet, peace and blessings be upon him, said: "Whoever takes people's money with the intention of repaying it, Allah will repay it on his behalf; and whoever takes it with the intention of destroying it, Allah will destroy him." This may intercede for him, but it does not absolve the heirs of the right to pay the debt from the inheritance before its distribution. Continue your efforts to settle the debt from the lands, or let your uncle bear responsibility for it.