Vayakom hasadeh vehame'arah asher-bo - The field and its cave was established
The Torah appears to repeat itself as we had been told already in v17-18 that the field and the cave passed from the possession of 'Efron to that of Avraham and the Torah specified its precise location. However, v17-18 speak about what happened to the field and the cave before Sarah was buried there. At that time the property passed to the hands of Avraham only as a miknah (a purchase). Now that Sarah had been buried in the CAve of Machpelah the Torah speaks of it becoming an achuzah, an inalienable piece of property. By burying Sarah in that cave Avraham had performed an act of permanent acquisition.