Sam observes two things the boy does before he and Bill grab him and pull him into their buggy.
One evening after sundown, we drove in a buggy past old Dorset's house. The kid was in the street, throwing rocks at a kitten on the opposite fence.
'Hey, little boy!' says Bill, 'would you like to have a bag of candy and a nice ride?'
The boy catches Bill neatly in the eye with a piece of brick.
These actions by the freckle-faced ten-year-old boy ought to have given Sam and Bill fair warning that he was going to be hard to handle. They also foreshadow the coming troubles for the benefit of the reader. Sam and Bill ought to have selected a more manageable victim. They obviously don't know anything about kids. What has them focused on this particular boy is the fact that his father seems to be a man who would have a lot of money. Also, the boy they have targeted is an only child. Sam, the narrator, has said:
We selected for our victim the only child of a prominent citizen named Ebenezer Dorset. The father was respectable and tight, a mortgage fancier and a stern, upright collection-plate passer and forecloser.
The boy, who calls himself Red Chief, turns out to be a holy terror. But his father Ebenezer is even worse. Instead of paying the two kidnappers the two thousand dollars ransom they demand, Ebenezer makes a counter-offer he knows they can't refuse. He will take Red Chief off their hands if they pay him two hundred and fifty dollars cash. In his note Ebenezer adds:
You had better come at night, for the neighbours believe he is lost, and I couldn't be responsible for what they would do to anybody they saw bringing him back.
O. Henry probably gave Red Chief's father the first name of Ebenezer as an allusion to Charles Dickens' famous character Ebenezer Scrooge of A Christmas Carol. Scrooge was a man who was capable of driving a comparably hard bargain.