Rainmaking

The conman represents that he (or he and his crew) can cause something to happen, for a fee, that will greatly benefit The Mark.

Actually, the event will take place or it will not. The conman has nothing to do with it. If the desired result does not come about, the most common play is to claim that what is needed is more money from the mark.

Comic Books

 * In an amusing reversal, The Rainmaker of PS238 is an actual mutant with the fairly lame power of making it rain or stop raining. He tries to make a living as a, well, rainmaker, but because of the countless frauds who have gone before him, nobody will pay him up front, and most of the time they turn out to be unwilling or unable to pay him afterwards - and as he puts it, he can't hardly pull the rain back outta the ground.

Literature

 * The Rainmaker (not the John Grisham one) has this as its central premise.
 * The Rainmaker (the John Grisham one) is a term in legal practice, though informal, for someone taking a case that seems all but worthless and making it pay out. A lot. The title rainmaker in the novel, Rudy Baylor, takes a case from two folks living on disability (a couple hundred a month) and wins the case for fifty million dollars. Hence, he took a drought (poor folks case) and made it rain money!.
 * This is of course the total inverse of the trope. Rudy makes it clear to his clients that he might lose, does not charge them any money, delivers the desired result and

Live-Action TV

 * In Season 3 of The Wire, a state senator solicits money from a drug dealer to be used to "grease the wheels" for getting a federal grant. The state senator just plain keeps the money, and the grant goes to whoever it would have gone to anyway.
 * The Twilight Zone episode "Mr. Garrity and the Graves" featured a con artist who claimed he could raise the dead. After "showing off" his work (with an assistant), he then offered to "reverse" it if the townsfolk paid him more. They did so, since everyone in the town's cemetery but one had died violently. In a Karmic Twist Ending, at the end, the con artist left... without realizing that he really had raised the dead, now en route to town.
 * In the episode "A Single Drop of Rain" of Quantum Leap, Sam leaps into the life of a travelling "rain maker" (who is, in fact, a con man) visiting a drought-stricken farming community. Sam decides to combine his knowledge of future cloudseeding techniques with an afternoon of yelling at God that He owes Sam big time, resulting in a beneficial downpour.

Real Life

 * Real Life example: Corrupt Church leaders, particularly televangelists, will often promise to cause miracles for or bring good fortune, wealth, and/or happiness to anybody who donates money to their church which, of course, they keep to spend on things like a $23,000 toilet. This is frequently done with The Shill acting as a benefactor of a supposed miracle, such as sitting in a wheelchair then suddenly standing up and walking, when they never needed the wheelchair in the first place.
 * A popular joke among NASCAR fans and broadcasters goes: "If your area is experiencing an extended drought, just build a racetrack and invite the NASCAR Sprint Cup Series to town." This is due to the unusually high number of rainouts NASCAR has had in recent years.