Stephen, as reported by Luke, in Acts, and the writer to the Hebrews, make it clear why this transaction means so much in regard to what was promised to Abraham, in the context of what he appeared to receive.
Stephen states that 'the God of glory appeared to ... Abraham, when he was in Mesopotamia' and as a result Abraham removed to Canaan, wherein :
he gave him none inheritance in it, no, not so much as to set his foot on: [KJV]
and promised to him to give it unto possession [literal]
επηγγειλατο αυτω δουναι εις κατασχεσιν αυτην [TR]
[Acts 7:5 KJV and TR, undisputed]
The promise was unto possession. But Abraham never possessed any of it and had to pay for a space in which to bury his wife.
Of Abraham, the writer to the Hebrews says, Hebrews 11:8-13 :
By faith Abraham ... went out not knowing whither he went ... By faith he sojourned in the land of promise as in a strange country ... These all died in faith not having received the promises but having seen them afar off ... confessed that they were strangers and pilgrims upon earth.
The promise, what it actually was, is made clear by Paul :
... for the promise that he should be the heir of the world was not to Abraham, or to his seed, through the law, but through the righteousness of faith. [Romans 4:13 KJV]
God was promising, throughout Abraham's life (look to the north, the south, the east and the west, to thee and to thy seed shall I give it) the whole earth for a possession. As far as Abraham could see to the horizon (about fifteen miles, at best) was not the promise. What God was promising was 'the north, the south, the east and the west'.
The inheritance, as is made clear in the New Testament, was a new earth : under new heavens.
And to thy seed (singular) is the promise, through Abraham and through David to Jesus Christ :
Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ. [Galatians 3:16 KJV]
Thus Abraham bought a grave in which to bury Sarah for he did not (and would never) gain possession of even a foot's breadth of land in Canaan.
For the promise was the world to come.