No Child Left Behind
"Why don't you put your money where your mouth is, Malfoy?"
"Scarhead, what do you propose?"
"I'll bet a hundred Galleons to your one, no wait, 100-1 odds are unfair... to you. I'll bet a hundred Galleons to half a Galleon that I'll catch the snitch."
"So I get a victory party and some pocket money. You're on."
"Wait. I don't trust you, you don't trust me. So," Harry held his wand up in front of him, pointing to the ceiling, "I swear upon my magic that if Draco Malfoy catches the Snitch I will pay him one hundred Galleons, no more and no less."
"Swearing upon your magic for something so petty?" Draco sneered. "Very well, I swear upon my magic that if Harry Potter catches the snitch I will pay him half a Galleon, no more and no less."
Ron waited until Draco had gone around the corner before he turned to Harry. "A hundred Galleons! Are you mental?"
"I got tired of breathing the same air as that wretched waste of space. All I have to do, Ron, is catch the Snitch and Malfoy loses his magic."
"No, he doesn't. All he has to do is pay you half a Galleon."
"No more and no less. How many Knuts are to a Galleon?"
"About 500. Why?"
"How many exactly?"
"I can't quite remember."
"493 Knuts to a Galleon. Now tell me; could you give me half a Galleon?"
"I don't have half a Galleon, you know that."
"Limited time to explain here Ron, 17 Sickles to a Galleon, 29 Knuts to a Sickle. Those two numbers are prime to each other meaning that you can only break a Galleon up into some multiple of 17 and 29. Two is not a multiple of 17 or 29 and therefore it is impossible to pay half a Galleon."
"I don't get it, but I'll trust you. What if the teachers make you cancel the bet?"
"Doesn't matter. The phrasing of the oath was that he would pay me half a Galleon, not that he would honor the bet."
"So all you have to do to get rid of ferret face-"
"Is win a game of Quidditch."
Author's Notes: I don't own Harry Potter.
Dark!Harry, though no more dark than the Marauders, but Sirius did try to kill Snape... Besides, in his time at Hogwarts Harry attacked Draco in the air (first flying lesson, Harry was the first aggressor), drugged students (Crabbe and Goyle, second year), stole from a professor (also second year), cast two of the Unforgivables, formed an illegal militia, killed one DADA professor, kidnapped another, and was complicit in the probable rape of a third (Umbridge and the centaurs, there was a Cracked article about it and everything).
This use of oaths is entirely fanon. With that said there are a couple ways for Malfoy to save his magic: he could catch the Snitch, he could cut a Galleon exactly in half (but the terms of the oath was 'pay,' suggesting a monetary value, a cut Galleon may or may not have that value. And the Galleon would have to be cut perfectly in half. Magic may or may not accept that solution), or he could tell a teacher (Snape) about the bet before the game and the Hogwart's staff could force Harry to let Malfoy catch the Snitch.
I had Hermione in the scene being horrified by what Harry had done, but then I realized she would go fetch a teacher and everything would be for naught.