How was Peyton Farquhar tricked by the scout into attempting to burn down the bridge?

Ambrose Bierce provides most of the background exposition in a flashback in Part II of "An Occurrence at Owl Creek Bridge." It can be seen in the interchange between Farquhar and the Federal scout disguised as a Confederate soldier that the scout is not really trying very hard to trick the planter into trying to burn down the bridge. Farquhar gets himself into trouble, and the scout just allows him to do it. It should not be assumed that the scout's job was to ride around trying to trick Southern civilians into deciding to commit acts of sabotage against the invading Union army. That doesn't seem sensible. Why should the Union army try to create further trouble for itself when it has enough trouble already? The scout is just a scout. His job is to collect whatever useful information he can, either through observation or through casual conversations with civilians. He is mainly interested in movements of Confederate troops and matters of that sort.

The scout specifically warns Farquhar that attempting sabotage could be very dangerous.

"The Yanks are repairing the railroads," said the man, "and are getting ready for another advance. They have reached the Owl Creek bridge, put it in order and built a stockade on the north bank. The commandant has issued an order, which is posted everywhere, declaring that any civilian caught interfering with the railroad, its bridges, tunnels, or trains will be summarily hanged. I saw the order."

This can hardly be viewed as encouragement to commit sabotage. Farquhar is described as s civilian who tries to serve the Southern cause in any way he can. Furthermore, Owl Creek Bridge is only thirty miles north of Farquhar's plantation. The whole Union army would soon be heading his way, which could mean looting his stores of food and freeing his slaves. He has a strong motivation to delay the Union advance by burning the bridge. He asks the scout what could be done to achieve this end.

The soldier reflected. "I was there a month ago," he replied. "I observed that the flood of last winter had lodged a great quantity of driftwood against the wooden pier at this end of the bridge. It is now dry and would burn like tinder."

The scout is only telling the bare facts and not suggesting anything. There is no more conversation between the two men because Farquhar's wife brings the water she had gone back into the house to fetch. Naturally the scout will inform his superior officers that a man might be attempting to set fire to the driftwood late that night. Farquhar will walk right into a trap, but it is his own fault. He is a romantic. He was trying to perform a deed of spectacular heroism by attempting to burn down a wooden bridge guarded by a whole contingent of Union soldiers. There were many men inspired by the ideal of "gallantry" who found out that war is nothing but vandalism and murder. Farquhar will remain a romantic idealist up to the moment his neck is broken by the noose.