וַיָּ֨קָם הַשָּׂדֶ֜ה וְהַמְּעָרָ֧ה אֲשֶׁר־בּ֛וֹ לְאַבְרָהָ֖ם לַאֲחֻזַּת־קָ֑בֶר מֵאֵ֖ת בְּנֵי־חֵֽת׃ (ס)
И поле, и пещера, которая в нем, были удостоверены Авраамом для владения местом погребения детьми Хета.
Rashi
your father, etc., and your mother Abraham and Sarah took their greatness from there, and the children of Heth gave her a grave.
Rashbam
לאברהם למקנה, “it arose” after Avraham had paid the money, a normal procedure which we are familiar with from the (misquote) in Erchin 33 ונתן הבסף וקם לו. The purchase was concluded in verse 18, i.e. the field became a מקנה for Avraham. It became an ancestral piece of property only after Sarah had actually been buried there, i.e. after the purpose of the transaction had been carried out (verse 20)
Ibn Ezra
AND AFTER THIS, ABRAHAM BURIED SARAH: After [the burial of Sarah] the field was made sure as a burial ground possession belonging to Abraham and his descendents. Torah tells [of the purchase of the field of Machpelah] ...it informs us that G-d's word to Abraham that he would possess the land as an inheritance was fullfilled.