Would it be Canned by Alex Shearer?
First hit on google when typing 'boy tin can factory child slavery finger book'.
Summary from Kirkus Reviews (emphasis mine):
This macabre mystery may make readers give up canned foods. Fergal Banfield, an eccentric English lad with the peculiar hobby of collecting unlabelled cans from supermarket bargain bins, discovers a gold ear stud in his latest acquisition. The mystery deepens when he next discovers a severed human finger in another can. Fergal meets Charlotte, a fellow can-collector, who finds a human ear in one of her cans, and it goes with the gold stud. Fergal’s investigation leads him to a pet-food factory owned by Mr. and Mrs. Dimble-Smith. Fergal makes the grisly discovery that the factory is staffed by enslaved young children from Africa and Asia, who become ingredients of the pet food when they grow old enough to resist captivity. Pressed into service and fearing for his life, Fergal gets a message to Charlotte on a can label about his situation and location, but no one believes her story. So it’s Charlotte to the rescue! The grotesque elements of the story are more suggestive than descriptive, and Shearer’s delightfully droll, dark humor makes for many light moments. Readers with a taste for the bizarre and gross will find this tale most tasty.
Thanks to @Mithrandir for adding some related extracts:
The Protagonist is a young male who collects unmarked cans.
Fergal felt he needed somewhere to hide, a wall to retreat behind - some shelter. He didn't want a hobby to bring him into contact with other people, he wanted one to shield him from then. And that was when he became interested in cans.
Not rare cans, though, nor ancient cans from expeditions to the North or South Pole; not foreign cans from exotic places, or cans in unusual colors. Just simple, ordinary, everyday cans.
Without labels.
Chapter 1
I believe they are cans of soup or dog food (not sure!).
"For you, Fergal," he whispered. "I'll open this one for you."
And he returned to the kitchen.
Mr. Bamfield fetched a plate and the can opener. With solemn ceremony he clasped the butterfly opener around the rim of the can...
...The smell of it alone was enough. Never mind the sight of it - that pinky-gray mush look it had. It was hard to know exactly what it was supposed to be. It smelled like some kind of economy-brand pet food.
Chapter 15
He has a habit of buying old cans and his parents are worried about him
Soon, to his mother's increasing unease, Fergal had filled the entire top shelf of his bookcase with cans.
Chapter 2
One day he finds another person who loves collecting unmarked cans as well (a young female).
"How do you know," he said, "that it's more important to you?"
"Because I need it," the girl said. "For my collection!"
Chapter 7
Eventually, he finds a decapitated finger in one of the unmarked cans.
It was a finger. A finger. A human finger. With a slightly grubby fingernail, and a mark around it - a faint indentation around the base.
There it was, lying on his desk.
One canned finger.
One finger, canned.
Chapter 5
I believe he finds a tin can with a wedding ring, a human ear and a note that pleads for help.
"It's an ear," he said. "Off someone's head."
Chapter 8
Fergal took the small gold earring from his pocket and placed it down on Charlotte's dressing table.
"It's a sleeper."
"That's right."
"Interesting."
"What did you find first?"
"This." Charlotte reached into her desk drawer, took something out, and placed it next to the gold stud.
"It's a ring."
Chapter 8
There were four of them: one vowel and three consonants. They spelled out the word h e l p.
Chapter 10
He realizes that all the cans come from the same factory and he decides to venture out and explore it himself
The code, Charlotte thought. He must have found the code. He must have found the code on a labelled can. he must have discovered where the strange cans were coming from. And there the manufacturer's name would be, on the label of the can he had found - a name and probably an address too. That must have been why he was so excited. He had matched the code.
Chapter 13
Eventually he finds the factory and finds that the owner of the factory is missing a finger.
"Interested in cans, are you?" he said. He held his hand up so that I could see he had a finger missing.
Chapter 15
The owner enslaves the male protagonist and he finds that the owner has illegally bought dozens of child slaves who are forced to work in his factory 24/7 in poor conditions.
Well, I bit and kicked and scratched as hard as I could, but I was overpowered and taken inside. I heard the door clang shut behind me and the sound of a double lock. And I've not seen daylight since. All I've done is work.
There are two shifts here - all day and all night...
...When we get to sleep, we're cramped together into two small rooms - one for girls, one for boys. Sometimes you have to wake somebody up to tell them that it's time for them to go to work so that they will get out and you can have their bed. The food we get is disgusting.
Chapter 15
The parents of the protagonist and the police searches for him for weeks but have no clue at where he is (the factory is in another town).
She was at the bus depot. She felt her feet take her to the information point; she heard her voice say, "Excuse me, where can I get a bus to Havverstock?"
Chapter 16
The protagonist (somehow) sends off a letter to the female tin collector in one of the tins hoping she finds it (which she does!).
But the biggest shock of all came when she found label one and began to read what was written.
It was a letter, addressed personally, to her.
Chapter 15
In the final chapters of the book, the female tries to tell authorities that he has been kidnapped and enslaved in the factory but as she is young no one believes her.
"Look, love," he said, "I'm sure you mean well. I've got kids myself, see, and I know what you're all like. They get fantasies, don't they? I used to get them too when I was your age. Dreams about being a hero, about saving people from drowning and getting medals and being modest about it and all the rest. Am I right?"
Chapter 17
She also ventures alone to the factory to try and save the protagonist and she does.
She jumped down from the step. The door closed behind her and the bus went on its way.
She was alone in the still of the night.
Chapter 16
All the slaves are released and the protagonist is back with his family.
The children had been taken away and looked after. Those who had homes were reunited with their parents. Those who were orphans were found decent families to live with. The factory was closed down.
Chapter 17
I believe the slave owner is mutilated by one of his factory machines because in the last chapter I remember it saying that no one ever heard again of the owner and in another tin can someone finds live human fingernails and toenails.
But the Dumble-Smiths had gone. How they had gone, where they had gone, nobody seemed to know.
Chapter 17
He fished it out and inspected it. It was a nail.
A great big toenail.
Chapter 17
