"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."
Alice has something Bob must have — it's a Matter of Life and Death — and makes Bob agree to an unreasonable trade before he can have it. Odds are good that Bob will regret it, even though Alice hasn't threatened to do anything to him. Read The Fine Print if you like — it's not like you have any power to make your objections stick. 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, or attempting to persuade the other character to engage in Releasing from the Promise. If there's a Magically-Binding Contract, though, you're in trouble. Sometimes a Deal with the Devil will take this form.
Super Trope of Boxed Crook. Close cousins with Read The Fine Print.
The name comes from Aesop's Fables, "The Lion and His Fellow-Hunters".
This term is occasionally used in contract law to refer to this kind of deal. At common law, such a contract is almost always considered unenforceable under the doctrines of unconscionability (i.e. the exchange was grossly unfair) and duress (which is more or less "the contract was leonine").
Examples
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Anime & 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. He makes her this offer while she happens to be dying from critical injuries sustained in a car wreck that occurred just moments before.
In Nth Man, Alfie cures the dying Sgt. Levin in return for a future service. Unfortunately for him, when he tries to collect by having her assassinate John Doe, she tells him to take it back, and he is apparently no longer in a strong position.
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.
In The Goose Girl, the princess promises the maidservant that she will not tell the truth in order to save her life.
Film
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.
Inigo: I want my father back, you son of a bitch.
In Tangled, Rapunzel gets Flynn to agree to guide her while she has him tied up and is threatening him with violence. Then, he has to guide her through her maniac-depressive mood swings. Unsurprisingly, he tries to weasel out any way he can except forgoing the tiara.
Literature
In The Bible, a starving Esau sells his birthright to his younger brother Jacob for some stew.
The Aesop fable "The Lion and His Fellow-Hunters", where the lion gets the lion's share (that term, which means all or nearly all of anything, also gets its name from that tale), because none of the smaller animals who hunted with him are able to argue the point.
In GK 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.
In The Da Vinci Code, Bishop Aringarosa tries to get a pilot to change landings to a different country, which is in violation of customs. When he offers the pilot one of the bearer bonds, worth about 10,000 Euros, the pilot only sees it as a piece of paper. However, the pilot is very interested in the Bishop's ring...
In The Count of Monte Cristo, Luigi Vampa and his men take Danglars prisoner and deprive him of any food except for what he buys from them at astronomical prices. This was, of course, masterminded by the Count as a means of separating Danglars from his ill-gotten wealth.
In Robert A Heinlein's The Cat Who Walks Through Walls competing rescue parties on the Moon attempt to force a stranded party into one, forcing the survivors to either accept the low bid or die on the lunar surface.
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.
Power Rangers Lightspeed Rescue: In order to make sure his son Ryan wouldn't die from a car accident, Capt. Mitchell agreed to let Diabolico raise the son until Ryan is 21.
Super Human Samurai Syber Squad: When Sam's little sister needed an operation, their family couldn't afford it because a Mega-Virus Monster had stolen money from theirs (and everybody else's) bank accounts. Sam's only hope was Malcolm Frink, who'd only help if Sam threw a pie at the face of the girl both of them like. Fortunately she understood and forgave Sam in the end.
Played with by The Roommate Agreement in The Big Bang Theory. It appears to be this, but with one important exception: Leonard could walk away at any time, and there is absolutely nothing Sheldon could do about it legally. (Emotionally, on the other hand...)
Web Comics
Mocked in Troops Of Doom, when the leader of the Imperials is charged 50 grand each for a grenade by the leader of the mercenary union because she is in the middle of a firefight and cannot get out alive without them. Also, said leader wasn't particularly happy with her disrespect for the union building's neutrality rules.
Girl Genius with its European Overlord. At least, Klaus was reasonable in his demands.
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!
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.
In Adventure Time,Finn sees Marceline abusing a poor old man who she claims is her minion, and he reluctantly agrees to take his place. Marceline then orders him to fulfill a series of evil orders that all turn out to be less evil than they appear. Eventually Finn realizes this is all an elaborate prank, so Marceline fires him.
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.
Robert Ringer tells the story of a man he worked for who was bargaining with a man who desperately needed the agreement as he was in bankruptcy and would lose everything. Knowing his desperation, the man forced him to accept really onerous terms on threat of walking away from the deal. The man comes up with the fees he has to pay, signs the agreement and after he leaves, Ringer mentions to his employer that he hoped the guy could keep terms of the agreement. His boss smiled and said, "He won't. If he'd bothered to read the contract he'd realize he was in technical default before he even signed it. I won't have to do a thing except keep the advance payment he made."
(i.e. the exchange was grossly unfair) and duress
