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.

Genesis 23:17 - The cave of Machpelah.

And the field of Ephron which was in Machpelah, which was before Mamre, the field, and the cave which was therein, and all the trees that were in the field, that were in all the borders round about, were made sure

This cave within the field of Machpelah was purchased by Abraham as a family sepulchre. It is the resting place for Abraham and his wife Sarah, his son Isaac and his wife Rebekah, his grandson Jacob and his wife Leah (Genesis 25:9; 49:31; 50:13), and perhaps others. The location of the cave is believed to be below the Haram el Khalil mosque, also called Abraham's mosque.

Genesis 23:11 - Beware of gifts, for nothing is free.

Nay, my lord, hear me: the field give I thee, and the cave that is therein, I give it thee; in the presence of the sons of my people give I it thee: bury thy dead.

Abraham was interested in only buying the cave of Machpelah; however, Ephron the Hittite offered to give him the cave, plus the field as well. Likewise, Araunah the Jebusite offered King David more than what he wanted to buy, all at no cost (2 Samuel 24:22). Both Abraham and King David refused to accept the gift and paid the fair market value of the offer. Solomon addresses the evils of gifts with the following proverbs:

• He that is greedy of gain troubleth his own house; but he that hateth gifts shall live, Proverbs 15:27.
• Many will intreat the favour of the prince: and every man is a friend to him gifs, Proverbs 19:6.
• The king by judgment establisheth the land: but he that receiveth gifts overthroweth it, Proverbs 29:4.