Napoleon has a few different dealings with Frederick and Pilkington, who own neighboring farms.
Probably the most important one (the one you're probably thinking of) is in Chapter 7. In that chapter, Napoleon has been negotiating with the two men to sell one of them a pile of wood that is left over from when Farmer Jones owned Manor Farm. Napoleon first says that he is going to sell it to Pilkington. But later on, he goes back on what he has said. He declares that Pilkington is an enemy and he sells the wood to Frederick instead.