In Genesis, we see that Abraham makes Ephron accept money for the Cave of Machpelah in order to bury Sarah. He is offered the cave for free, but insists on paying the full price. This sort of story seems to be trying to teach us a lesson, so what is the lesson that we are supposed to learn?
Is it that we must pay money for burial plots? Is it that we should never accept gifts for free? Maybe that we shouldn't accept gifts from non-Jews? Something else?
I am looking for sourced answers that teach us what the lesson is.