"Then Abraham rose and bowed down before the people of the land, the Hittites. He said to them, 'If you are willing to let me bury my dead, then listen to me and intercede with Ephron son of Zohar on my behalf so he will sell me the cave of Machpelah, which belongs to him and is at the end of his field. Ask him to sell it to me for the full price as a burial site among you.'" NIV translation
Abraham replied to their generous offer by asking to buy the cave of Machpelah. We see from this that he had already done the leg work and found the spot. Surely, he was led there by God and it reminds us of the fact that we must be listening to God in order to be led by Him.
"Ephron the Hittite was sitting among his people and he replied to Abraham in the hearing of all the Hittites who had come to the gate of his city. 'No, my lord,' he said. 'Listen to me; I give you the field, and I give you the cave that is in it. I give it to you in the presence of my people. Bury your dead.'" NIV translation
The owner of the cave was listening and offered to give not only the cave but also the field to Abraham.
He was not only giving Abraham the cave but also the field which would have been giving away his food or income.
"Again Abraham bowed down before the people of the land and he said to Ephron in their hearing, 'Listen to me, if you will. I will pay the price of the field. Accept it from me so I can bury my dead there.'" NIV translation
Abraham insisted on paying for the land and the cave but we do not know why. Abraham was a wealthy man so he would have had plenty of money to buy it but the Hittites were trying to give him a gift.
"Ephron answered Abraham, 'Listen to me, my lord, the land is worth four hundred shekels of silver, but what is that between you and me? Bury your dead.' Abraham agreed to Ephron's terms and weighed our for him the price he had named in the hearing of the Hittites: four hundred shekels of silver, according to the weight current among the merchants." NIV translation
The negotiations settle on a price and Abraham counts out the money. So, why did he not accept the gift? We see the answer in Abraham's previous dealing with the king of Sodom (Genesis 14). He knew that his wealth had come from God and not men. He did not want any man to be able to take credit for the blessing that God had given him.