Fan Works / You Said You Would Let Them Go

Between My Brother and Me: After May's duel with Mai, she demands that Dartz release Yugi's soul as part of their deal, Dartz replies that he can't, as May only specified that she wanted a way out, and hadn't made any further bargains with him.
Cracked Glass: When Shota betrays the Resistance, they tell Katsuki that he was promised that the children from their farm would be spared, not anybody else.
In Fear the Superhero, a teenaged apostle tries to pull a Take Me Instead in order to save his girlfriend. Karasuba offers to let both go free, only for Shirou to immediately snipe both of them.
I'm okay, even if I leave my grin behind has a possible example in the Backstory, depending on who you believe. According to Master Fu's version of the story, Gardamon had promised to hand over all of the Dark Miraculous after they teamed up to take down Lucifer, but reneged on his promise afterward. According to Gardamon, he'd only promised to let Fu claim the Black Cat Ring, which was lost when Lucifer cast away the Ring and the hand bearing it to spite them. Fu then tried to steal the rest of Gardamon's collection, but was caught in the act.
Raise Your Voice Against Liars: Bob Roth intends to screw over his latest targets in this fashion. After kidnapping Juleka Couffaine, he plans to demand a ransom from their family, but has absolutely no intention of returning his victim, instead selling them off to somebody in Germany and shipping them away.

Forced to Be a Family: Asami is only willing to work with the Equalists so long as they promise that they'll leave Bolin, Mako and Korra alone. When they decide to take the trio to Amon instead, she breaks them out.

In Lost and Found, John Gage convinces a deranged killer to take him instead of Roy-but then the guy shoots Roy anyway after making John cuff him to the squad. Poor John spends 18 months of captivity thinking this and occasionally saying something close to it, believing Roy is dead.

Harry Potter and the Perversion of Purity: Harry asks Draco to let Neville and Hermoine smuggle the dragon out of Hogwarts so that Hagrid won't get into trouble. Draco claims that he won't go through with his plan, only to immediately betray his trust and tip off the aurors, getting Hagrid arrested.

Kage: It's revealed in Miranda's backstory that Kur lured her to hand herself over to him by saying that her parents would be spared from being burned at the stake as part of his shapeshifter purge. Unfortunately, Kur then vetoed his promise by asking the like-minded crowd's opinion on the matter, leading Miranda's parents to be killed anyway.

In the High School AU Hit List, Colin tips off Ganondorf as to where certain students will be at the time of his attack, hoping that the bargain will let them save Ayrll. Unfortunately, the very person they're trying to protect happens to be somebody Ganondorf is targeting, as he's aiming to take down Link and those Link cares about the most. So he never had any intention of sparing them once they weren't required as leverage anymore.

Quicken: When a group of thugs assaulted Emma, she let one of them cut her nose off because their leader Lao promised that they'd let her go. When Lao showed that he had no intention of keeping his promise, she protested. When he said he lied, she went completely berserk and killed them all:
Emma: Y-y-you promised. You s-s-said that – that you would let me go after – after she was done.
Lao: Guess I lied, ginger.

You Said You Would Let Them Go / Fan Works