Is it permissible to annul the sale and return the amount paid due to the buyer's failure to pay the full agreed-upon price within the specified period, and is the buyer required to complete the payment at the new land price if he still wants it, or does he deserve a profit if it is sold to someone else, knowing that there are no official documents between the two parties?

Installment sales are permissible, but the mentioned contract included a corrupt condition, which is the late payment penalty, considered prohibited interest (riba). This condition invalidates the penalty itself, but the contract remains valid. Therefore, your relative's purchase of the land is valid, and you do not have the right to annul the sale except with his consent. You may demand the agreed-upon price (42500), and your relative must pay if he is solvent; otherwise, he must be given respite until he becomes solvent.