What is ironic about Tom Sawyer helping Huck Finn "free" Jim even though he already knew that Miss Watson freed him in her will?
This is at the end of the book. When Tom helps to create but also complicate the plan to free Jim.
The irony lies in the fact that Tom and Huck have grown up knowing nothing different than white people own slaves. Jim, having traveled with Huck, has established a friendship and has proven to Huck that black people are just like white people except for the color. They think, feel, love, hate, and desire to be free and live happily ever after with their families. Tom has not had this experience. Tom would not be so quick to help Jim escape if he didn't already know Jim had been legally freed by the Widow Douglas--it is too ingrained in him that slaves are owned and escaped slaves are turned in to the law. Huck, on the other hand, decides to help Jim escape regardless. Tom will likely never consider black people as actual people to be considered fairly and humanely.