Why does Huck say that Jim has been tricked and mistreated for ""forty dirty dollars""? What makes the money dirty?

It was an illegal money, made by conning

Explanation:

At the end of Mark Twain's The Adventures of Huckleberry Finn, the slave Kim was freed by Huck Finn, making him, finally, a free man.

However, he was tricked by two con men they met along the journey, and sold back into captivity for 40 dollars, making him a slave once again. The money is dirty because he comes from a con and was made by selling a human being as property when the seller had no right to do it in the first place, even taking into account the slave culture at the time.