How can the price of 10 meters of land, agreed upon for purchase with a deferred payment, be estimated? Should it be based on the previously agreed upon price or its current market value?

There is no correlation between the price of the remaining square meters and the price of the land that was previously sold. If you wish to purchase these meters, you must inform the owner and agree with her on the price, as an agent is not permitted to buy for himself what he was appointed to sell, except with the permission of his principal.