Chayei Sarah 23:3-4
3 And Abraham arose from before his dead, and he spoke to the sons of
4 "I am a stranger and an inhabitant with you. Give me burial property
with you, so that I may bury my dead from before me."
I am a stranger and an inhabitant with you: [I am] a stranger from another land, and I have settled among you. [Consequently, I have no
ancestral burial plot here (Rashbam, Sforno).] And the Midrash Aggadah
(Gen. Rabbah 58:6) [states]: If you are willing [to sell me burial
property], I am a stranger, but if not, I will be as an inhabitant and
will take it legally, for the Holy One, blessed be He, said to me, “To
your seed I will give this land” (above 12:7).
Rav Hirsch points out on verse 3 the source for the law of onein:
he only left his dead to make arrangements for it; hence our sages lay
down the law that as long as the dead be unburied; the relative is to
think of nothing else than attending to the business of seeing that
the dead receives its due. Until the burial is duly carried out, this,
and nothing else at all, is to receive all his attention.
Rav Hirsch continues on verse 4:
Abraham does not ask for permission just to bury his wife. He wants
his wife to rest in her permanent everlasting possession of her
resting-place, that is why he first asks for the right to acquire a
piece of land as freehold property for the purpose of a sephulcre.
Rav Hirsch continues on verse 19 that this particular location is requested because it was also the burial place of Adam and Eve, the progenitures of all mankind and, therefore, had a particular holiness. The cave (Machpelah - doubled) had locations for pairs to be buried so that he would be buried with Sarah. There were other pairs so that Yitzchak and his future wife and the heir of Yitzchak and his future wife could also be buried there.
Therein lies the thought that the particular place where it was wished
that the burial was to be made had special value for the one who was
to be buried there, or for those who belonged to him. It says that he
is not just to be buried, but to be buried there. Now tradition says
that Sarah was not the first to be buried in that cave. Adam and Eve
already rested there. The parents of mankind were the first and they
are to be joined by the parents of the Jewish people, those spiritual
parents of mankind, that was why Abraham chose just this cave.