Why did the man steal the protagonist's lump?
According to Wikipedia:


  An old man has a lump or tumor on his face. In the mountains he encounters a band of tengu making merry and joins their dancing. He pleases them so much that they want him to join them the next night, and offer a gift for him. In addition, they take the lump off his face, thinking that he will want it back and therefore have to join them the next night. An unpleasant neighbor, who also has a lump, hears of the old man's good fortune and attempts to repeat it, and steal the gift. The tengu, however, simply give him the first lump in addition to his own, because they are disgusted by his bad dancing, and because he tried to steal the gift.


The mean old man doesn't want to steal it, he wants to get rid of it like the protagonist did.