What is the ruling on a land seller demanding an additional price years after the initial sale, claiming that the original price was undervalued, and is it permissible to take this money?

If a sale fulfills its pillars and conditions, it becomes binding on both parties, and neither has the right to unilaterally annul it, as Allah Almighty says: "O you who have believed, do not consume one another's property unlawfully but [only] that your property may be [exchanged] by mutual approval from you." Administrative documentation is not a condition for the validity of the sale, and therefore the seller is not permitted to stipulate an increase on the original price, because the land has exited his ownership.

Nevertheless, it is possible to agree with the seller on a specific amount for his travel expenses to carry out the documentation, because he is not originally obligated to do so, and he is allowed to take compensation for what he bears.