When Sam and Bill are arguing over the amount of money to demand of Ebenezer in their ransom note to him, Bill uses a metaphor to capture why he believes Johnny is worth less than $2,000, as seen in the following passage:
“I ain’t attempting,” says [Bill], “to decry the celebrated moral aspect of parental affection, but we’re dealing with humans, and it ain’t human for anybody to give up two thousand dollars for that forty-pound chunk of freckled wildcat. I’m willing to take a chance at fifteen hundred dollars.”
By referring to Johnny as “that forty-pound chunk of freckled wildcat,” Bill metaphorically compares Johnny to a ferocious animal (a wildcat) while, at the same time, making it clear that the boy is not exactly a force to be reckoned with (as he is only a “chunk” weighing 40 pounds). This captures Bill’s ambivalent feelings toward Johnny—while Johnny is actively violent toward Bill (leaving him with numerous bruises), Bill also sees clearly how Johnny is just a kid. Bill knows that the boy doesn’t actually mean him any harm, leading him to be oddly patient with Johnny over the course of their time together. It’s likely due to Bill’s empathy (however imperfect) that Johnny doesn’t want to go back to his less emotionally available father at the end of the story.
In addition to these poignant undertones, there is also some classic O. Henry humor in this scene—while Bill shows some amount of care for this little “wildcat,” he is not actually objecting to using him for ransom; he merely thinks they should lower the amount to $1,500.