All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
What is understood from the question is that your father really sold the land which he had promised to the farmer and that you received part of its price. Therefore, if the situation is as you mentioned in the question, then the land had become the property of the farmer and there is no way of getting him out of it. Hence, it is not a matter of fulfilling or not fulfilling the promise, since the land had become his and no longer become your father’s. What should be looked at now is the remaining amount from the price of the land. If your father had exempted him in his life while he was not so sick that it was feared that he would die because of his sickness, then the farmer is exempted from the remaining amount. Indeed, the jurists stated that whoever acquits his debtor from his debts even before it is due, with the wording of ‘you are free from this debt’ or ‘this is a charity for you’ or ‘this is a gift for you’ and other expressions that mean renouncing the debts from him, or abandoning it, or making it possessed by him, or forgiving it for him, then the debtor becomes acquitted. Therefore, you have no right to ask the farmer to pay the remaining amount.
However, if your father did not acquit him from it, but he just promised that he will renounce it in the future, then the farmer is obliged to pay that amount of money and you have the right to take it.
Nonetheless, there is a last probability, which is that if your father had acquitted the farmer during his death sickness, or that he had conditioned acquitting him from the money on his (your father’s) death, then this kind of acquitting takes the ruling of a will, in which case the farmer is acquitted from the money if it is worth a third of the inheritance or less than a third, but if it is more than a third, then the farmer can only be acquitted from what equals the third.
Allaah Knows best.