Disclaimer: You can tell I'm not JKR because she has hair and I shave my head.
Harry sat in the dank darkness. Assuming he was fed twice a day, he'd been here at least six months. Maybe more, a lot more. In the beginning the dementors had affected him really badly and he'd spent a lot of time unconscious, trapped inside the nightmare of his mother dying.
Dementors weren't so bad now. Harry could have given in to the nightmares. It would have been so easy. Give in to the madness and be taken away from the dark and the dank and the dementors.
He wouldn't do it.
Giving in to the madness would mean giving in to the corrupt ministry which put him here. Instead, Harry dug deep into his mind and his magic and found a power he hadn't realized he had. This let him resist the dementors' aura.
Harry knew the prophecy. Unless Voldemort managed to take over the ministry, they'd eventually need Harry to put down the dark lord.
The Azkaban guards tormented Harry daily. They almost seemed offended that he hadn't gone mad yet. "Hey, murderer, why don't you make friends with some of the other prisoners? Then you can kill them, too, and save the dementors some work."
Harry took strength from the taunts. As long as the guards thought he was a bad person because he had killed his friends, then it meant the ministry hadn't fallen to Voldemort yet.
Days rolled over to days, marked only by meals; no sun could reach this far into the prison's depths. And on one of the days came some new voices.
"Potter! Wake up there!" Shacklebolt, ever the lackey, shot stinging spells into the cell to call attention to his boss.
"Potter, in a show of our mercy, the Ministry will give you one chance to atone for your acts." Somehow Fudge was still Minister of Magic. Somehow the tubbo managed to sound pompous even here.
"Finally realized you need me, did you? Took you long enough."
"Watch your mouth, murderer!" Shacklebolt sent more stinging hexes at Harry. They didn't do anything. No doubt this looked very impressive to those outside the cell, showing that Harry could catch magical attacks on his bare hand. The truth was less impressive: winter in a freezing cell had given him frostbite. He couldn't feel a thing in his left hand.
"We do not need you. It would be convenient to have you perform a service for us. We have come to make you a one-time offer of a binding contract to release you from confinement in exchange for your service."
Dumbledore finally spoke up. It was amazing that the old babblemouth had kept quiet this long. "Harry, I'm sure that if you look deep in your heart, you will find the innocent boy who wants to do the right thing. ..."
Harry stopped listening but didn't interrupt. He used Dumbledore's endless yammer for time to think. Dumbledore hadn't changed at all, but he had always looked a thousand years old. Shacklebolt wasn't any balder, of course, but he looked somehow older. Tireder and more beaten down. Fudge was fatter and there were worry lines despite the rounder face. Things must be going badly for them, just as he'd expected.
When there was finally a chance for him to get a word in, Harry demanded, "Tell me about what you want me to do, and what I get for doing it. And you'll want to be quick about it because the dementors will be coming again in a few minutes."
"Yes. Well. Er, you will be released from prison and given a wand, and then you will go out and destroy Lord Thingie."
"What else? What about after? Do you plan to just chuck me back into Azkaban? Oh, no, I know. You'll have the aurors kill me because I escaped, right?"
Fudge looked surprised. Whether it was because he had never thought of such a plan or because Harry had figured out his plan was anyone's guess. Harry knew which way he was guessing.
"Of course that was not the intention, Harry." Dumbledore was the very image of reasonableness. "If you feel it necessary for your peace of mind, we can add a paragraph to the contract to pardon you for your earlier crimes. We can do nothing about the court of public opinion. You will simply have to lead an honest, productive, and non-violent life to convince your fellow wizards and witches of your good intentions."
"Fine, whatever." The opinion of the sheep didn't matter to him. He'd empty his vaults and leave as soon as they took their eyes off him. "What about my property? My wand and Firebolt and vault key and other things? When do I get that back?"
"Your possessions were confiscated and sold as compensation for the families of your victims and to reimburse the ministry for expenses in apprehending and confining you."
"You. Stole. My. Inheritance?" Harry was furious. "So you frame me, throw me in here without a trial, and steal everything I own, and now you think I'll take care of Voldemort" - wait for the flinch - "for you? Go to Hell! You can give me back all of my things and then we can talk."
"I'm warning you, Potter, this is a one-time offer. We are much too busy and important to come here again if you do not accept this today."
"Nothing doing, Fudge. You need me more than I need you. Make a real offer or get out of here."
"Your life here could get a lot worse, Potter," Shacklebolt said venomously. "All I need to do is have a few words with a few friends of mine. Sign the contract or else."
"Ha! You wouldn't be pushing so hard if you weren't desperate. Every day I wait, the price goes up. Every threat you make, the price doubles."
Dumbledore, ever the voice of moderation, cut in. "Now, Harry, I'm sure Kingsley didn't mean that. We are all under a great deal of stress and you have to make allowances. Look over this contract and see if we can arrive at an acceptable compromise."
"You can take that contract, fold it until it's all sharp corners, and shove it where the sun don't shine."
Dumbledore sighed, disappointment writ large on his face. "Come, Cornelius, Kingsley. Harry's in no mood to be reasonable." They all headed toward the exit.
Fudge grumbled, "I suppose we'll have to take care of the problem ourselves."
"Indeed, Cornelius. It will be more expensive but it can be done," Dumbledore reassured him. "When they write the epic story of this war, we'll each have books about us but Harry's part will take only two more lines." Their voices faded in the distance.
Harry waited for them to return, sustaining himself with ever more extravagant demands that he would make in exchange for his help. Eventually he died and his body was thrown into the sea.
Author's Note: Inspired by the beginning of DobbyElfLord's "Harry Potter Mercenary" (story 4544334 on FFN). I like the story, but the setup is rather questionable. Harry must have been brain damaged to sign that contract.