Is a seller entitled to revoke the sale of five feddans before the contract is written, and what is the ruling if the buyer has benefited from the land for twenty years without a written contract for the remaining five feddans, nor a contract for the other fifteen feddans?

If the land was state-owned and your father utilized it for 20 years, there are three possibilities: Either the state permitted him to utilize it (as a grant), in which case there is no sin upon him, but he does not own it, and the state can reclaim it. Or it was barren land that he revived through legitimate cultivation, and thus it becomes his property. Or he utilized land that was not barren, and he did not cultivate it, and the state did not permit him, then this is an aggression.

Regarding the sale that occurred to your father from a person who rightfully owned the land from a governmental entity and was paying its installments, the sale between your father and the seller became binding, and neither party can revoke it except with the consent of the other party, whether the contract was written or not, unless there was a reason for an option, such as a defect, or an excessive imbalance in price, or a condition. The essence of a contract lies in the offer and acceptance, and writing is merely documentation. Therefore, neither the seller nor his agent has the right to revoke the sale of the five feddans, and your father can adhere to the transaction as it is. There is no sin upon him for utilizing all the land during the previous period. If he is prevented from disposing of or utilizing the five feddans after the completion of the purchase, there is no sin upon him for utilizing the remainder, and the sin falls upon whoever prevents him from utilizing his right.