What is the lumber deal that napoleon makes?

The correct Napoleon sells the lumber to fredrick for 5 pound notes.

A pile of timber lies unused on the farm, left over from the days of Mr. Jones, and Napoleon engages in complicated negotiations for the sale of it to either Mr. Frederick or Mr. Pilkington. When negotiations favor Mr. Frederick, the pigs teach the animals to hate Mr. Pilkington. When Mr. Pilkington then appears ready to buy the timber, the pigs teach the animals to hate Mr. Frederick with equal ferocity. Whichever farm is currently out of favor is said to be the hiding place of Snowball. Following a slew of propaganda against Mr. Frederick (during which Napoleon adopts the maxim “Death to Frederick!”), the animals are shocked to learn that Mr. Frederick eventually comes through as the buyer of the timber. The pigs talk endlessly about Napoleon’s cleverness, for, rather than accept a check for the timber, he insists on receiving cash. The five-pound notes are now in his possession.

Soon the animals complete the construction of the windmill. But before they can put it to use, Napoleon discovers to his great outrage that the money Mr. Frederick gave him for the timber is simply a stack of forgeries.