Debt Detester

"Just this one time, I let you go. For the little girl. You and me, we're even then. No more owed. Understand?"

—Thresh, The Hunger Games

So Bob owes Alice big time. Maybe she saved his friend, or did something selfless for him, or risked her life for his, it doesn't matter. The thing is, Bob and Alice aren't friends. They might even be enemies. Where this trope comes in to play is when Bob inexplicably helps out Alice, usually followed by a "Now we're even" from Bob. The key to this trope is that it's a one-shot deal; if Alice gets herself in trouble later it's her problem, not Bob's. Bob may even be the cause of her troubles.

Contrast I Owe You My Life, a more long term, friendly, and annoying variation of this. Unrelated to Work Off the Debt. Occasionally overlaps with Because You Were Nice to Me, albeit rarely.

Examples of Debt Detester include:

Anime and Manga

Yu-Gi-Oh!: Kaiba claims that Tea helping Mokuba escape from the Rare Hunters is the only reason he's helping Yugi find his friends. Then he uses it as his reason for saving Tea's life. Considering all the times he makes a point of explaining how he's only "helping" the gang out of selfish reasons, it's pretty easy to say it's only a shield for his Hidden Heart of Gold.
In Arakawa Under the Bridge, the protagonist has always lived by this principle, paying all debts as quickly as possible, but he finds himself in a quandary when a crazy woman who lives under a bridge saves his life.

Comic Books

Elf Quest: This is ostensibly Rayek's reason for helping Leetah to save Cutter's life after the disastrous battle with the snow trolls. In reality he probably did it out of compassion, though he'd never admit it.

Film

Shang in Mulan spared the titular's character's life after she dishonored the Chinese army in return for her saving his life during the Huns' attack.

Literature

The Hunger Games:
- Thresh spares Katniss' life because she had been nice to Rue.
- Katniss herself is an example of this trope: when The Hunger Games starts out, she hates Peeta because he defied his mother and gave her bread when she was starving. After Thresh saves her, she thinks:

I nod because I do understand. About owing. About hating it.

The Marquis de Carabas in Neverwhere
The Lannisters in A Song of Ice and Fire.
One Discworld book (Carpe Jugulum I think) says that this is a key part of witch psychology and that if you really want to hurt a witch then you need to do her a favour she has no way of repaying. The unfulfilled debt will always nag at her.
The faeries in The Dresden Files, although it's not so much that they dislike being in debt as that if they make bargains they're compelled to keep them.
In Harry Potter, James Potter saved Snape's life and it's implied that Snape protects Harry because he regrets not being able to save James' life in return. Or at least that's the story in the first few books, until it's revealed that he also loved Lily.

Live-Action TV

Shran on Star Trek: Enterprise, vis-a-vis Archer.
In Star Trek: The Next Generation, Q. In one of the early Q episodes Picard shelters Q after Q has been made mortal temporarily. Q hates that he owes Picard a favour and tries to get Picard to name a favour so he can resolve the obligation.
- Of course, Picard knows that Q is The Trickster and a Literal Genie and refuses to accept repayment. Q's response is to make up a need of Picard's and 'solve' it for him in an incredibly roundabout and dangerous way.

Western Animation

From Gargoyles, after Macbeth helps Demona kill Gillecomgain:

Demona: Thank you.
Macbeth: I owed you.
Demona: Then we're even; good.

Other Media

Entry 68 of the Evil Overlord List on This Very Wiki states -- "I will spare someone who saved my life sometime in the past. This is only reasonable as it encourages others to do so. However, the offer is good one time only. If they want me to spare them again, they'd better save my life again."