Genesis 23:20 - To make sure the possession of land.

And the field, and the cave that is therein, were made sure unto Abraham for a possession of a buryingplace by the sons of Heth.

A more detailed example of how land was purchased and made sure is given in Jeremiah 32:10:

And I subscribed the evidence, and sealed it, and took witnesses, and weighed him the money in the balances.

And I subscribed the evidence is the writing of the deed to record the details of the transaction. and sealed it the deed was rolled, tied and sealed to be kept as the permanent record. and took witnesses refers to those that witness the negotiations of the transaction; in the case of Abraham's purchase, they were the sons of Heth. And weighed him the money in the balances because coined money was not yet used, the silver was weighted out on a scale.