How does Mr. Frederick double-cross Napoleon?
Napoleon had been reluctant to deal with Frederick because he wouldn't pay Napoleon's asking price. Somewhere along the line in their agreement, Napoleon came down or Frederick came up.
In truth, after the money was exchanged, it was discovered
The bank notes were forgeries! Frederick had got the timber for nothing.
This was noted just after the animals had a celebration about the notes and Napoleon put them on display for the animals to admire. When it was discovered that these were fake, Napoleon was made a fool, but he didn't lose heart, he braced for an attack from Frederick.
It arrived shortly thereafter and at the Battle of the Windmill, the windmill was blown to bits by Frederick and his men.
Frederick pays Napoleon for the timber with counterfeit bills in George Orwell's novel, Animal Farm. At first claiming that Frederick is the animal's greatest enemy, Napoleon suddenly makes a deal with him, selling him the timber instead of Pilkington. It is Napoleon's greatest moment, but three days later, it is discovered that the bank notes are forgeries. Frederick gets the death sentence from the animals, but the next day Frederick and his men attack in what became the Battle of the Windmill. The animals retain the farm, but Frederick's men destroy the windmill--and keep the wood for free.
The way that Mr. Frederick double crosses Napoleon is by cheating him on the deal where he (Frederick) was supposed to buy the woodpile from Napoleon.
Napoleon had been negotiating with Frederick and Pilkington to try and get the best deal possible. Finally, he decided to sell to Frederick. At first, Frederick is going to pay by check, but Napoleon figures that that is a trick and forces him to pay cash.
So Frederick brought a bunch of money that he used to pay for the wood. As it turned out, though, the money was all fake.