Wager Slave

Betting is appealing, both in Real Life and in fiction. It adds a sense of risk to otherwise normal proceedings, makes everything more interesting. Unfortunately, "gambling is fun" is a poor aesop, not to mention that it's hard to make money actually mean anything in a work of fiction. So there's an easy solution: Rather than the loser giving the winner money, the loser has to obey the winner's orders—either once or for an entire day.

This almost always involves Unresolved Sexual Tension, though good luck seeing it resolved. Unless it's a porno. Guess how that ends. Cousin trope to Lost Him in a Card Game. Compare Trapped by Gambling Debts

Examples of Wager Slave include:

Anime and Manga

Touma and Mikoto in A Certain Magical Index set up one of these when their schools face off in a sports festival. Mikoto wins, and wonders what she should make him do. Eventually, she basically makes him go on a date with her (although she doesn't tell him that), which gets interrupted by random super-powered sociopaths. So, you know, an average day.
In The World God Only Knows, Haqua challenges Keima to a board game, with a Wager Slave as the penalty. As it becomes increasingly clear that she is hopelessly outmatched, Haqua starts saying, "I know I said anything... But not anything! Just not anything, okay!" Keima, however, only wants to ask questions about his situation and Elsie. Haqua finally lashes out at him: "Aren't you even a tiny bit interested in me?!"
The Melancholy of Haruhi Suzumiya has Haruhi invoke this on whoever is late (usually Kyon). In a slight subversion, he just has to pay for everyone's lunch.
In the early part of the Yu-Gi-Oh! manga, "other Yugi" is all about penalty games...doubtful if there's any sexual tension involved.
- Joey does this with Duke. Duke wins and puts Joey in a dog suit, forcing him to obey all his commands—until Yugi steps in to free him. It's not treated kinkily in the anime, but that hasn't stopped fans from taking it that way.
In Gantz, Kurono is forced to date a non-hottie as a result of one of these. The sexual tension is, in fact, resolved.
In a breather episode of Soul Eater just after the Kisin had been revived, the main cast play one of these in a three-on-three basketball game. If Kid's team loses, then they move all the pictures in his house slightly off; if Maka's team loses, she has to spend the weekend with her father. Maka loses and her Bumbling Dad is so happy he vomits.
Higurashi no Naku Koro ni has the main characters' club often handing out these kinds of penalties to the loser of the game of the day. One memorable result of this is Keiichi being forced to walk home in a maid outfit.
- This is a rare one where there is no sexual tension, but there may be extremely violent murder involved when things inevitably go wrong.
Comes up in Star Driver.

President: If you win, I'll double your pay. If you lose, you have to kiss me through the glass...except without the glass, and for as long as I want.

In Marmalade Boy, Ginta plays in a tennis tournament against his cousin, Tsutomu Rokutanda, who states that the loser has to shave their head. Of course, he ends up losing, and does go through with shaving his head.

Film

The main subplot in Hackers is a bet between Dade and Libby, with the loser ending up as the loser's slave or date respectively. Either way, they have to wear a dress.

Literature

At some point in the Hitch Hikers Guide to The Galaxy series, Ford Prefect's interior monologue reminisces about a drinking game which, if you lost, you had to perform a forfeit, "usually obscenely biological".
In Four Clues for Rani, during a treasure hunt game, Rani bets Vidia that if her team finishes first, she [Rani] will do whatever the fast-flying fairy wants for one week, and vice versa.

Live-Action TV

At least twice on The Brady Bunch:
- Greg bets Marcia that he's a better driver than she is, with the stakes being the loser does the winner's chores for a week. Greg loses.
- Greg bets Bobby that he can do twice as many chin-ups as Bobby can, with the stakes being that the loser has to do whatever the winner wants. Greg loses again. The result is that he has to do Bobby's chores for a week.
How I Met Your Mother has the famous "slap bet," where Barney and Marshall get in a bet over what Robin's deep dark secret is. When Barney's guess of "porn" appears to be right, he slaps Marshall... but when it turns out that was in error, the restitution is that Marshall gets to slap him at 5 random points in the future.

Western Animation

An episode of Tracey McBean involved a dare in which someone had to ride down Mount Massacre. The loser had to be the winner's slave for a whole week. It did sort of border on Foe Yay territory.
Clone High does this when the principals of two schools competing in a basketball game decide that the losing principal has to do the winner's laundry...for a week.
The Simpsons: Homer bets Ned that whoever's son doesn't win a minigolf tournament has to mow his lawn in his wife's Sunday dress. Due to Homer enforcing Exact Words, both of them end up doing it. Homer gets pissed off when Ned enjoys it, recalling his college fraternity prank days.
Happens on The Venture Brothers: Dean dares Hank to drink some awful substance, and in exchange Dean will be Hank's slave.
The Looney Tunes short "Early to Bet" does this. The Gambling Bug bites a cat, and the cat is compelled to bet with a Bully Bulldog, with the payments in the form of penalties or forfeits.
In The Grim Adventures of Billy & Mandy, Mandy offered to play a game with the Grim Reaper. If Grim wins, he gets the hamster he sought. Grim's ego led him to offer that if he loses, he would become Billy and Mandy's best friend. Unfortunately for Grim, the hamster jumped into his eye socket making Grim lose a game of limbo. Grim becomes their best friend.
This is the plot point and reason for the South Park episode "The Red Badge of Gayness". It's averted at the end.
At the start of the My Little Pony Friendship Is Magic episode "The Ticketmaster", Applejack tells Twilight Sparkle that she bet Big Macintosh that she could get all the golden delicious apples picked and put in the barn by lunch time, and if she does, he has to walk down the street in one of Granny Smith's girdles.
- Then, in "Applebuck Season", Big Mac is unable to help her harvest the rest of the apples due to a mysterious back injury (which isn't really that mysterious if you consider the difference in size between Big Mac and Granny Smith). Let this be a lesson to you, children: when you gamble with your sister, make sure public humiliation is the only stake.
This provides the plot for The Angry Beavers episode "You Promised!", when Norbert promises Dagget he'll do this in a year in exchange for the last "Lickety Split" candy in the house, not expecting Dagget to actually remember his promise.
In an episode of Lilo and Stitch, Jumba and Pleakley get caught up in Lilo and Stitch's argument over who's better at capturing experiments, and each agrees to coach one of them in capturing a pair of experiments. They agree to bet that whoever's protegee captures the experiment first, the other has to perform "the Funky Glognart, the insipid dance of a thousand hours". Meanwhile, Lilo and Stitch learn a lesson in teamwork and end up capturing both experiments together, which means both Pleakley and Jumba have to perform the dance.