Bava Metzia 108 - The Right of the Adjoining Property Holder
An immediate neighbor of a property has the right of first purchase to it. An outsider who wants to buy a field can easily find another one, but a neighbor gains significantly from being able to buy an adjoining field. This rule is based on " You shall do what is right and good in the eyes of God …" which commands one to go beyond the letter of the law.
If one wants to buy a field and inquires of the neighbor of that field if he can do it, and the neighbor allows it, a formal act of acquisition is still required, or else the neighbor can later claim that had he come to buy the land, he would be asked a higher price, but that he never meant to relinquish his right.