In his will, the uncle Mr. Wilks said he was leaving the girls $6,000. When the Duke and Dauphin count the money, however, it comes up short around $415. The Duke and Dauphin decide to use their own money to make up the difference, believing that this will confirm to everyone that the Duke and Dauphin are who they claim to be, the English relatives of Mr. Wilks.
"Good land, duke, lemme hug you! It's the most
dazzling idea 'at ever a man struck. You have cert'nly
got the most astonishin' head I ever see. Oh, this is
the boss dodge, ther' ain't no mistake 'bout it. Let
'em fetch along their suspicions now if they want to --
this 'll lay 'em out."
The two con men give the money to Mary Ann, and in spite of being warned by Dr. Robinson that these two men are frauds, that they have phony accents, etc., without hesitating whatsoever, the gullible Mary Ann gives the money back to the con men to invest. The irony is that later on, thanks to Huck, the two con men lose all of the money, including the $415 of their own that they added to the pot.
Although the two con men are crooks, Mark Twain is also very critical of the victims in this novel. They bring disaster on themselves due to their own gullibility and messed up morality. As you probably know, morality is turned topsy turvy in this novel. It is one of the themes. Whenever Huck and Jim are in society, the morality is all messed up. When they are on the raft, however, Huck seems to return to his inherent good morals. Although he thinks he is a sinner, he is really much more moral than the "civilized" characters.
Read about it here on eNotes.