Sarah died at 127 years old. This would have made Isaac around 36 years old when his mother died since she gave birth to him when she was 91.

Abraham was not only a righteous man, but he was also a righteous businessman. When he was negotiating for a cave and land to bury Sarah, Abraham wanted possession/control of the cave and the piece of land. Abraham did not want to use the cave, he wanted to own the cave and was willing to pay whatever price the cave was worth.

He did not want to be in debt to anyone or owe any man anything. This reasoning caused him to refuse to take a gift from the Hittites.