Why is the reply from Red Chief's father an example of irony?
The boy in this story was kidnapped. In a standard kidnapping, the family of the victim must pay to get their beloved returned to them. That's certainly what the kidnappers thought was going to happen in this story. Remember that the father is rich (at least in the kidnappers' eyes), and that they plan to ask the father for $2000 to return him?
What happens when they send their demand? The father insists that they (the kidnappers) will pay him to accept Red Chief back. That's exactly the opposite of what they expected to happen. It shows that the situation is not as they understand it to be--and it shows that the father knows very well how the kidnappers have gotten in over their heads.
A ransom is usually paid in one direction, but in the father's response, it will flow the other way. That's an anti-ransom, which is pure irony.