Leonine Contract

""I should not agree with your young friends," said Marcus curtly, "I am so old-fashioned as to believe in free contract." "I, being older, perhaps believe in it even more," answered M. Louis smiling. "But surely it is a very old principle of law that a leonine contract is not a free contract. And it is Hypocrisy to pretend that a bargain between a starving man and a man with all the food is anything but a leonine contract." He glanced up at the fire-escape, a ladder leading up to the balcony of a very high attic above. "I live in that garret; or rather on that balcony. If I fell off the balcony and hung on a spike, so far from the steps that somebody with a ladder could offer to rescue me if I gave him a hundred million francs, I should be quite morally justified in using his ladder and then telling him to go to hell for his hundred million. Hell, indeed, is not out of the picture; for it is a sin of injustice to force an advantage against the desperate." "

- G. K. Chesterton, "The Unmentionable Man"

Alice and Bob are making a bargain, but Alice has something Bob must have—it's a Matter of Life and Death—which means she has a free hand. Odds are good that Bob will regret it, even it's the condition of Take My Hand, and not a Deal with the Devil, even though Alice will not do anything violent, or even against him, without an agreement. The lack of any active threat makes it a favorite of the Manipulative Bastard.

Occasionally the bargain is reasonable, even if Bob doesn't want to admit it. This will strongly characterize Alice as just and reasonable. On the other hand, this trope often results in the partner with the advantage constantly Moving the Goalposts. The Revenge-minded may taunt a criminal with this before revealing that since the harm was irrevocable, so too is their desire for revenge; villains are prone to teasing with this.

Even Incorruptible Pure Pureness characters may hold The Promise void if it was part of this. Everyone is prone to insist on Exact Words. If the contract is magical, that's even worse, it's a Magically-Binding Contract.

Super-Trope of Boxed Crook.

The name comes from Aesop's Fables, "The Lion and His Fellow-Hunters". Sometimes used as a legal term; if the contract is too oppressive, it may be void, legally. (Also called an unconscionable adhesion contract.)

Anime and Manga

 * In Puella Magi Madoka Magica, Kyubey offers Mami the chance to Make a Wish, which he will grant in exchange for her becoming a Magical Girl..
 * In My-HiME, many a HiME is offered her powers under circumstances in which refusal would be deleterious.

Comic Books

 * In Nth Man, Alfie cures the dying Sgt. Levin in return for a future service.

Fairy Tales

 * "Rumpelstiltskin" and its variants: the girl will be killed if she doesn't get his help, which gives her no way to haggle when he demands her child.
 * The Three Spinners and The Three Aunts also technically have one, but the spinners only ask for Honorary Aunt status and an invite to the wedding.

Film
"Inigo: I want my father back, you son of a bitch."
 * The Princess Bride has a Revenge variant. Inigo Montoya has confronted the man who killed his father and has him at swordpoint. Knowing the man's cowardice, he makes a series of increasing demands as a condition of sparing Rugen's life. Of course, what he really wants is impossible, and so he never had any intention of actually making a bargain.

Literature

 * The fable "The Lion and His Fellow-Hunters", where the lion gets the lion's share (that term also from that tale), because none of the smaller animals who hunted with him are able to argue the point.
 * In G. K. Chesterton's "The Unmentionable Man", in The Paradoxes of Mr. Pond, this is alluded to—see page quote—and this is a clue to the man's identity.
 * In John C. Wright's The Phoenix Exultant, Ironjoy is in this position toward the Afloats; they can't even afford to report his crimes against them because they need him. He assumes that Phaethon needs him, too, which is unwise on his part.
 * In Robert E. Howard's "The Scarlet Citadel," one of Tsotha's slaves taunts Conan the Barbarian with the keys when he's prisoner, to ask what he would pay, as if to offer this. Finally he reveals that Conan killed his brother during his pirate days, when he was known as Amra, and that he will settle for nothing less than Conan's life.
 * In A Song of Ice and Fire Jaime Lannister vows not take up arms again against the Starks or Tullys while held captive, drunk, and with a sword against his throat. Despite this, the Kingslayer takes it seriously.

Live-Action TV

 * Occasionally referenced by the presenters on Top Gear: when a presenter's vehicle inevitably breaks down in an awkward place (like while floating in a harbor), another might jokingly offer to tow them to safety for a million quid, if they don't just abandon them.

Web Comics
"Klaus: One rule, Beetle. I made one rule when I left you this city. "Report any unusual discoveries. Devices of The Other are to be turned over immediately." You agreed. Dr. Beetle: A pledge made under duress is worthless, Wulfenbach! You threatened my city, my university -- I'd have agreed to anything! You were in control then! Klaus: And now? (while his clanks point at Dr. Beetle with machineguns about the size of himself)"
 * In Impure Blood, Roan is contemptuous of Dara, until she reveals she has the key to his cell.
 * Girl Genius with its European Overlord. At least, Klaus was reasonable in his demands.

Western Animation

 * In Tripping the Rift, the Devil forces Chode into selling his soul in exchange of rescuing his ship from four simultaneous black holes that have suspiciously appeared out of nowhere. His crew travels in time to hire the legendary lawyer Webster who defeated him in the past, but end up with the kid actor who played television's Webster instead. Being used to deal with agents and lawyers, the kid rapidly found a major loophole in the contract, claiming the deal was signed under duress. The Devil denies duress, blurting out that there were no real black holes, thus dooming his case as he never filled his side of the agreement. Chode walked away scott-free.

Real Life

 * William the Conqueror justified the Norman Conquest because Harold had sworn fealty to him—while he was his prisoner, as a condition of release.
 * A frequent objection against such organizations as the International Monetary Fund, which will loan money to poor and desperate countries - provided that they do things like hand over their water system to a private monopoly, slash labour and environmental regulations, sell off government assets at bargain-basement prices, introduce free trade agreements which may disadvantage them, et cetera. The IMF defends this as being in the countries' own best interests, but most of the people in said countries don't agree.
 * Agreements to receive unemployment payments such as Jobseekers Allowance in the UK. A 'Jobseeker' has to sign a list of commitments to look for types of work and available hours which may not be physically possible for them. However, not signing means no money, which means no food and shelter, which means eventual death, or at least hospitalisation for malnutrition or depression.
 * In fairness, there exists a dedicated system that evaluates how much a Jobseeker can realistically apply for and do. If someone is unable to work because they are an addict etc, then ultimately that is their decision (especially given that treatment is free and can be accessed at any time).