I would agree that the common theme is greed. I would add the cliche that if it sounds too good to be true, it probably isn't. What's more is that Pahom was warned in a dream to not fall for the deal. His greed takes over and he loses his life. The Baskirs realize that people are greedy and will try and cover more territory than can be covered in a day. Pahom is no different. In the end, his greed kills him.
The common theme is greed, Pahom's curiosity is piqued everytime he hears of additional lands, as he believes that if he had enough land, he wouldn't fear the Devil himself. Yet we see that more problems accompany more land, and at the end, we see the sardonic answer to the question posed by the title- ultimately, Pahom only needs the six feet of land he is buried in.
Greed that would lead a man to take ownership over more land than he can practically take care of is a greed that also leads to failure.
Everyone wants to grab up property if it is being given away, of course, and it is easy to believe that a change of scenery will make all the difference. Yet, as we see in the story, we can be easily drawn into mistakes of judgement when our ideas are founded only in loose dreams and in commonplace greed.
Every time something sounds good, it turns out to be not so good. For example, it sounds like the 40 acres of land is good. However, the other peasants' cows venture onto the land and it turns into a burden.