The story ends with a friend of Irving and Sydney redepositing $2 million into the FBI’s account in exchange for immunity for the couple. The FBI had transferred the $2 million into the friend’s account, wrongly assuming it was controlled by the mob.

 

I originally found the ending problematic. The mechanics of the final transaction did not seem to make sense, as the sender would have no leverage over the FBI once the money left his account. But the idea is not as counterintuitive once put into context.

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The FBI has little reason to bring new charges (or to fail to dismiss the old charges) against the couple. The two already cooperated in trapping a number of criminals, i.e. they lived up to their end of the bargain. In fact, the number of people arrested as a result of their cooperation actually exceeds the four they had originally promised. The government would have to drop their current cases and indict them anew on charges having to do with the $2 million the FBI mistakenly and stupidly transferred into their account – money which they returned almost immediately upon receipt. Not having to retrieve the money, the FBI would have nothing to gain but to embarrass itself by indicting the conning couple. Through this scheme, Irving and Sydney essentially put themselves in a position to leverage their silence. The money was returned, no one lost anything, and no one was hurt.

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So it does actually make sense for the couple to return the money (without a firm commitment from the other side). The ending works.