Jim’s evasion was spoiled by the fact that Jim was actually free, so there was no need to escape.
Tom Sawyer knew that Jim was a freed slave the whole time. Alas, he thought it was more fun to play with him and “help” him escape than to let everyone know. Tom would never have participated in Jim’s escape had he not already known Jim was free. Instead, he turned it into a game.
Always ready for a good time, Tom decides to have some fun with Jim’s escape. Huck has no idea that Jim is free. He honestly just wants Jim to get away. He trusts Tom Sawyer and looks up to him, so he goes along with the absurd plans.
When a prisoner of style escapes it's called an evasion. It's always called so when a king escapes, f'rinstance. And the same with a king's son; it don't make no difference whether he's a natural one or an unnatural one.” (Ch. 39)
Tom concocts one elaborate plan after another, until he finally has to admit that Jim has been freed. He has had the letter with him the whole time. Jim can’t be a captured escaped slave because he never was an escaped slave in the first place. Miss Watson freed him.
Tom’s antics are a perfect example of Twain’s biting satire. We find it funny and not funny at the same time. We worry about Jim, we feel sorry for Huck, and we are exasperated by Tom. The whole thing is designed to get the readers to realize that slavery is wrong, no matter how you dress it up.
The evasion fails to occur in the last chapter of the book because Tom and Huck discover that Jim has been set free, and no longer desire to occupy Jim's time. Tom wants to send Jim down the river in a raft where they can continue having their adventures. Then he would tell Jim that he was actually free, and they would return home up the river in a steamboat. Instead, they give Jim forty dollars for playing the part of prisoner for so long and so well, and Aunt Sally and Uncle Silas dote on Jim for aiding the doctor in nursing Tom back to health.
The first time I catched Tom private I asked him what was hisidea, time of the evasion?—what it was he’d planned to do if theevasion worked all right and he managed to set a nigger free thatwas already free before? And he said, what he had planned in hishead from the start, if we got Jim out all safe, was for us to runhim down the river on the raft, and have adventures plumb tothe mouth of the river, and then tell him about his being free,and take him back up home on a steamboat, in style, and payhim for his lost time, and write word ahead and get out all theniggers around, and have them waltz him into town with a torch-light procession and a brass-band, and then he would be a hero,and so would we. But I reckoned it was about as well the way itwas.We had Jim out of the chains in no time, and when Aunt Polly andUncle Silas and Aunt Sally found out how good he helped the doctornurse Tom, they made a heap of fuss over him, and fixed him upprime, and give him all he wanted to eat, and a good time, and noth-ing to do. And we had him up to the sick-room, and had a high talk;and Tom give Jim forty dollars for being prisoner for us so patient,and doing it up so good, and Jim was pleased most to death.