An Archie Comics story had Betty accidentally knock her diary into a box of items that she was donating to the local auction. Once she found this out, she tries to bid it back when it gets presented at the auction the next day, but loses to Veronica who's anxious to find out what Betty had written in her diary. Of course, she gives it back to Betty after being called out for it by Jughead.
At the end of Rat Race, when chasing for the prize everyone was looking for, they end up in a charity marathon, and everyone believes they're donating the prize. Although they eventually decide to go ahead and donate the money for real.
Animorphs: the trope is invoked when the Animorphs need to find a Helmacron spaceship (which looks like a child's plastic toy) in a Goodwill station. As the girls have no idea how to describe it, the boys claim it's a Romulan Bird of War, and belonged to Jake. However, the employee is himself an SF fan, so he warns them they'd better come out of there with a Romulan ship.
Diary of a Wimpy Kid: The Heffleys are in church and Greg gets some money from his mom to put in the collection plate. He's so busy trying to look saintlike for the girl sitting behind them that he fails to notice it's a twenty, and then he tries to make change.
During The Office episode "The Alliance", Michael Scott donates twenty-five dollars to Oscar, whose nephew is participating in a cerebral palsy walk-a-thon. Michael doesn't think twenty-five dollars was a large amount, so it's odd how everyone is congratulating him for it, until he realizes that the sum put in is dollars per mile, and the walk-a-thon is 17 miles long. He spends the rest of the episode agonizing over whether he should lower the amount. He doesn't.
Mad About You: Paul accidentally donates his & Jamie's bed to Goodwill and has to track it down to buy it back before Jamie finds out.
The Brady Bunch: Marcia hides her diary in a box of books in the garage, which Cindy then gives to a book donation drive.
Doug and Carrie are at a charity event that involves picking leaves off a fake tree that list an item for the person to buy to furnish a family's house. They pick one without realizing how expensive it will be, and Hilarity Ensues as Doug tries to put it back without drawing attention.
Doug and Carrie donate money to a library. Their names are listed with individuals who donated a certain amount of money; unfortunately, it's in the wrong category. Hilarity Ensues as we get to see how many times they try to correct this, and Carrie actually admits that she's willing to be generous, but really petty.
An episode of Spin City features the mayor saying he'll give an ill boy his "kidney" instead of the "key to the city".
In the Good Luck Charlie episode, "Blankie Go Bye-Bye", Teddy accidentally puts Charlie's blanket in the donation pile instead of the laundry pile. She tries to get it back from Leslie, a fellow charity employee, but he refuses to give it to her. She even sneaks into the back of his truck to look for it. It is later revealed that Leslie used Charlie's blanket as a seat cushion because he has posterial hydrosis.
On an episode of The Love Boat, it's the captain's 5th anniversary as the captain, and the crew hires two men to make him a wooden replica of the ship as an anniversary present. Just as they're finished (re)building it, one of the men accidentally drops his watch inside the model. At the anniversary party, the captain is quite pleased with his gift — and what a unique box it came in!
In Yakuza 4 during a revalation moment, an old homeless man accidentally drops a 500 yen coin, and chasing after it, knocks it into a donation box being held by a girl. Said girl then thanks him and hands him a small candy while the homeless guy is in despair over losing the money.
In a Home Movies episode, Coach McGuirk accidentally gave a homeless man more money than he meant to and then swapped it for the smaller bill he meant to give. For the rest of the episode the homeless man follows him around publickly shaming him for taking his money back.
In an episode of Dan Vs., Dan accidentally donated his car to the Salvation Armed Forces and they refused to give it back.
A heart-warming example in a Christmas Episode of 6teen, where the gang lose their presents and eventually find that someone put them in the donation bin. They decide to leave them there.
In the Family Guy episode "A Very Special Family Guy Freakin' Christmas" Peter accidentally donates the family's presents along with the actual donations box.
This has happened a few times on The Flintstones. Fred or Barney hides money (once in a bowling ball, once in an old suit) and Wilma or Betty gives the item away.
Phineas and Ferb: Doofenshmirtz's ex-wife accidentally donated a doll Doof had gone through some trouble over years to acquire for Vanessa, because it fell into the donation box.
A woman dropped a diamond wedding ring into a homeless man's cup. Because she gave him a little something each day, he decided against pawning it. Instead he gave it back to her when he saw her again. After that, she and her husband-to-be set up a fundraiser for him, in which he gained more than $100,000. He subsequently started a business, gained living quarters, and reunited with his estranged family. A happy ending all round!
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it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.