Stolen Good, Returned Better

Bob has stolen something from Alice. And That's Terrible. Bob then proceeds to upgrade, repair, or otherwise improve his stolen goods. But somehow—whether by Bob returning it, Alice stealing it back, or a random coincidence—Alice gets her stuff back. While she might have been unhappy about the theft, she's pleasantly surprised at Bob's improvements.

Anime and Manga

In Mobile Suit Gundam Wing, after OZ get their hands on some of the gundams, they are rebuilt just in time for the heroes to take them back.
In an early episode of Pokémon, Team Rocket steals a Ditto belonging to the professional impersonator Duplica, only to discover that it cannot mimic faces properly. They promptly whip it into shape, and when Duplica and the protagonists show up to steal it back, Duplica gives them heartfelt thanks for helping Ditto overcome its problem.
In Code Geass, after Kallen and her Guren are captured by Britannia, they strap a crapload of high-tech upgrades onto it only for her to break out and steal it right back. Rakshata, its creator, is surprisingly not happy about the Britannian's modifications, though they did come from her principal scientific rival.

Comic Books

The old Marvel Comics series The New Warriors played with this a bit. The character Silouette stole an Unstable Molecule costume from Reed Richards (she could meld into the shadows for teleportation but would always lose her clothes in the process since they can't meld with her, forcing her to find something with less Fan Service.) She shed her old costume and stole the new one, feeling guilty about the situation afterwards. She tried to return it to Richards, but he was fine with the situation and ended up tweaking it slightly for her... and offering to make further costumes for her in the future.

Film

In Back to The Future, Marty McFly steals a kid's scooter in the past, ripping off the handles so it's just the board. When he gives it back, it's the world's first skateboard.
- He does something similar in the future, ganking a hover-skateboard from a little girl. By the time he's returned it after getting away from Griff, she's already taken Griff's high-end, rocket-powered skateboard for herself.
In The Rocketeer, Cliff and Peevey don't steal the rocket from Howard Hughes, but Peevey's reverse-engineering improves its acceleration and also designing a helmet which improves flight greatly impresses Hughes.
In Amelie, Amelie "borrows" her retired father's garden gnome and gives it to a stewardess friend of hers with the instructions that the stewardess take photos of the gnome at famous landmarks and mailing them back to Amelie's father, in a effort to get him out of his doldrums. This is something that does happen in real life, and the movie helped popularize the practice.

Literature

In the Wyatt novels by Garry Disher, whenever Wyatt steals a car, he always leaves it in a parking lot with a full tank of petrol.
Harry Dresden of The Dresden Files once said that he got a note from a prospective carjacker offering to steal him something better. This is because his Blue Beetle is The Alleged Car; Harry can't drive anything more modern because of his magic, which shorts out anything that was invented after the integrated circuit.
In How the Grinch Stole Christmas, this is what the Grinch claims he's doing when Cindy Lou Who catches him, saying "I'll fix it up there, and bring it back here." By the end, it's only half-true, since he returns all the Christmas items, but hasn't really improved them.

Live Action TV

The Sopranos: Dr. Melfi is having trouble with her car, and with the mechanics who are fixing it. When Tony learns this during one of his therapy sessions, he has her car stolen, fixed, and returned that night. She's grateful, but not happy about it.
Played with in this segment from The Unpleasant World of Penn & Teller. Stephen Fry reluctantly parts with his expensive watch, which Penn & Teller proceed to improve. Specifically (and in order), the watch becomes waterproof, shock-resistant, smashed to bits, capable of holding fifty phone numbers, and fish-proof.
An episode of Las Vegas has Mike borrow his boss Big Ed's convertible to take on a date. While out, the car gets stolen by Mike's date, and Mike tracks it down to a chop shop. When they tell the man in charge whose car it is, he apologizes, has it put back together, detailed, and returned, except the radio favorites which Ed yelled at Mike about.
In an episode of Michael Bentines Potty Time, a thief steals Nelson's Column, the Effiel Tower and the Statue of Liberty. It turns out the thief was actually having them cleaned as a gift to the governments of the UK, France and the USA and returned them in 'as-new' condition.

Tabletop Games

In In Nomine, this is the modus operandi of Kyriotates, angels who manifest by possessing human bodies. (And animals, and in some cases plants and machines.) They suffer dissonance if they leave the host in a worse condition than when they "borrowed" it, so they'd rather be safe than sorry and put in some extra effort. Their demonic counterparts, the Shedim, couldn't care less about their host and take pleasure in corrupting it.
The Star Wars Saga Edition RPG supplement Starships of the Galaxy recommends this as a possible route for upgrading a player owned ship while avoiding the exact details on cost and time.

Video Games

In Chrono Trigger, Dalton steals the Epoch. But when the heroes steal it back, it can fly.
In Fallout 2, your car may be stolen in Reno, but if you find it in the chop shop, the guys who stole it will helpfully return it to you better than before.
In the classic text-adventure game Zork, one of the treasures you find is a golden egg... but if you let the thief steal it, when you find his lair later, you find that the egg has been opened, and it contains a golden singing bird—much more valuable!

Western Animation

In Teen Titans, the Hive takes over the Titans' (filthy) Tower. By the time the Titans retake it, the Hive have cleaned it up and alphabetized their CDs. Beast Boy and Cyborg are appalled, since now they can't find anything, and they were saving that blue mold.
Played for Laughs in a Futurama movie. Bender swaps Leela's engagement ring with a fake, but after examining the stolen ring, he realizes that the replacement he made is actually more valuable. He shrugs it off, considering it his gift to Leela.
- And again when Fry gets his hands back from the Robot Devil.

Fry: And what did you do to my nails?
Beelzebot: I cleaned them.

In Hanna-Barbera's The Little Rascals episode "The Case of the Puzzled Pals", Darla asks Alfalfa to help her find her missing doll. It turned out that Spanky had taken Darla's doll so he could put a new dress on it.
In Transformers Animated, the Headmaster steals Sentinel Prime's body and, while he has his hands on it, upgrades it.
In A Pup Named Scooby-Doo Episode 22 "Night of the Boogey Biker" (the first episode of Season 3), Scooby and the gang try to find out who stole Red Herring's aunt's bike. They discovered that Red took it to add a sidecar to it as a gift for her.

Web Comics

In Freefall, Sam steals his neighbor's truck, claiming it to be "borrowing" — he did intend to return it, after all. Florence works on it for a while before returning it. It runs a bit better afterward.

Real Life

An Urban Legend says Mister Rogers' car was stolen, and when the thief discovered that that was whom he robbed, returned it after detailing it.
- Snopes covered it here.
There have been incidents of stolen lawn ornaments like gnomes, eventually returned to the owner with a book of photographs showing the gnome in distant and exotic locations, if the photographs aren't sent from the location itself. The practice was a key part of the movie Amelie, as described above, which helped popularize it.
"Broomhandle" Mauser 96 pistols were copied in large quantities without license in Spain and China between the World Wars. Some of the copies improved on the design with added automatic fire and support for box magazines, which Mauser added to their own production in response.
Early production of the American Bazooka was captured from the Soviets it had been supplied to and reverse engineered into the German Panzerfaust which fixed a lot of the Bazooka's issues reliability and low stopping power while also weighing 5 pounds less. The Panzerfaust's improvements were implemented in the Soviet RPG-2 (which led to the Jihadist favorite RPG-7) and American Super Bazooka.