"The Devil and Tom Walker" is a picture of greed, as you mention. Tom Walker is a man who sells his soul to the devil in the next world in order to get more money in this world. He's miserly and crusty and shriveled, as is his wife. When the devil takes her, Tom is secretly thrilled she is gone. Once he's been given the riches he bought so dearly, he makes even more money. At some point, though, in the midst of his greediness and grasping for money, he has some kind of epiphany and decides to get his spiritual house in order. He seems to have undergone some kind of reform--until someone makes him angry. He makes a casual remark which brings him face to face with the devil, who has come to retrieve the soul he was promised.
Your theme of greed is clear, and a thesis which includes that would be accurate. Another may concern the idea that appearances can be deceiving, and what seems to be is not always what is. Tom is supposedly a God-fearing man at the end of the story, yet he ends up in hell with the devil. He appears to love his wife, yet he's relieved that she's gone. He seems to have gotten rich on his own, but he actually bought those riches with his soul. He doesn't want to believe the devil will really claim what he's owed, yet he practically invites him to come get him. What seems to be, may not be.