Bereschit 23:18 Kommentar: Rashi, Rashbam, Radak & Siftei Chakhamim

בכל באי שער עירו BEFORE ALL THAT CAME IN AT THE GATE OF HIS CITY — In the midst of all them, and with all of them standing by he gave him legal possession of it.

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)

לעיני בני חת. He weighed the money for Efron in the presence of all the assembled people, so that Efron sold the cave and the field to Avraham also in the presence of all his townspeople. He also handed Avraham a document confirming the sale.