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.