Is it permissible to revoke a verbally agreed-upon pledge of allegiance to a person who has delayed completing the purchase procedures for over three months, and to sell to another person offering a higher price?

If the remaining owners of the house authorized your friend to sell it, and he sold it to the aforementioned person with a genuine sale, and they separated physically from the session of the contract, then he has no right to retract; for the Prophet, peace and blessings be upon him, said: "The two parties to a transaction have the option [to annul it] as long as they have not separated." Official documentation is not a condition for the validity of the sale, and the seller may not sell what he does not own. However, if what transpired was merely an agreement and a promise to sell, then your friend should not break his promise, but selling to someone else is valid if there is a need for it and the first buyer delayed.