"Then Abraham rose from beside his dead, and spoke to the Hittites, saying, I am a resident alien among you; sell me a burial site among you that I may remove my dead for burial." How are we to understand the fact that Abraham waited until his dead lay before him and only then sought a burial plot, and did not foresee the need to prepare a site in advance? In my modest opinion, the reason is because he was not permitted to purchase a site in the Land of Israel so as to fulfill the commandment of being a foreigner. For God had commanded him to be a foreigner in the Land, and not to purchase any of its land, for it had already been given to him in potential and would be bestowed upon him in practice when the time would come. Had he purchased any land there, he would have transgressed what he was commanded and appeared to be of little faith.
Now, however, that Sarah had died, he could purchase the Cave of the Patriarchs (M'earat HaMachpelah) for her, the reason being that "death releases from obligations". Since she had died, she was released from the commandment of being a foreigner and immediately allocated a burial site.