Or HaChaim on Genesis 23:9/אור החיים על בראשית כ״ג:ט׳

1 ויתן לי את מערת המכפלה, So that he may give me the cave of Machpelah. Abraham made a point of describing the transaction as a gift though perfectly aware that money would be changing hands, as indicated by the words "for full monetary value." Abraham was astute enough to purchase the cave in such a way that any future complaint was out of the question. He was also careful to add the words: "in your midst." The Zohar 1,128 points out that Efron never saw anything in that cave except darkness and calamity. This being so, Efron could have claimed later that in view of his having been unaware of the true value of the cave, his consent to sell it was based on a false premise, and he could have reversed the sale. This in spite of the rule that there is no validity to a claim of land having been over or underpriced at the time of sale. Abraham still worried about an idea expressed in the Jerusalem Talmud (compare Tossaphot Kidushin 42) that if the undercharge was 50% or more the sale is reversible although it is not reversible when the amount of the undercharge was between 15% and 50%.

2There was also the concern that landowners of the adjoining property could claim that the land should have been sold to them in preference to an outsider, something known in Jewish law as דין המצר. Another consideration of Abraham was that after the transfer of the cave someone might present a lien dated prior to the transaction which would require him to give up his claim. For all these reasons Abraham used both the expression of "gift" and "sale" when referring to the way he wanted to acquire the cave. This would protect his claim also under the heading of what is known in Jewish law as דין חליפין, i.e. acquisition by means of a symbolic transfer. Under this heading there is no such law as אונאה, reversal of the sale due to unfair pricing. This דין חליפין applies to transactions involving either real estate or chattels whereas such acquisition by a symbol representing the ultimate object does not apply to money, i.e. coins. Hence Abraham's reference to receiving the land as a gift. Our sages in Bechorot 50 phrased it thus: Rabbi Chaninah stated that every time the word "כסף" i.e. silver or money appears in the Torah, the meaning is "סלעים", 25 silver coins of a standard denomination; the exception is the use of the word "shekel" in connection with the transaction involving Abraham and Efron. In that instance the meaning is קנטרין, a coin weighing four times the usual weight of the "shekel." This is the meaning of the Torah (Genesis 23,15) defining the "shekel" as being "four hundred shekel acceptable at any merchant." The fact that Efron arbitrarily changed the usual definition of the value of the "shekel," made the transaction unassailable in the future seeing he himself had set the terms of the sale. Concerning Abraham's fear that the דין מצר might be invoked by a neighbour in the future, Abraham ensured the presence and consent of all the local populace who would jointly act as his broker in the transaction. This would preclude any claim at a later date. It also demonstrated that there was no outstanding lien against the property as any such lienholder should have spoken up at that time, seeing he was present.

3Abraham used the term אחוזת קבר, "a burial plot held in perpetuity," leaving Efron the option to use the remainder of the field if he so chose. Had Abraham not used the term אחוזה, Efron could have denied him access to Sarah's grave once he had buried her there.