How did Avraham actually acquire the Me'aras ha'Machpeolah?
Torah Temimah (citing Kidushin, 2a): From the fact that Avraham said here "Kach mimeni!" and the Torah writes later
Why did Avraham say "Nasati" in the past tense?
Rashi #1: He meant the money was ready and he wished that he had already given it to him.
Rashi #2 (above, 14:22): He meant to say simply 'I am giving you the money for the field; take it from me!''
Ohr ha'Chayim: If you resolve to give the field for a gift, also I resolved to give the money for a gift! Take from me, and then I will accept your gift and bury my Mes.
Why does he say "Ach", which always limits?
Mizrachi: I do not want a gift, like you said.
Ohr ha'Chayim: Let us minimize words.
Malbim: Since you explain (refer to 21:8:1:3*). that the enactment was so only residents can have a cemetery, the only solution is for me to buy it. If it will be a gift, you could retract, and say that you did not give with intent that I make it a cemetery. Regarding a sale, we say that amidst the money, the seller resolved to transfer ownership (no matter what the buyer will do with it).