The foreign exchange(i.e., currency) market is both very liquid and very volatile. With advance knowledge of major changes from a Time Turner, it's easy to make very fast exponential growth of your seed money - even a 1% change per day, in any direction, multiplies your money by a factor of 12 every year(or x3 million before he graduates Hogwarts). Of course, anyone with that sort of record would get investigated hard and fast, but a more cautious approach can still result in absurd growth.

Let's quote the current author's notes:

One thing I did notice was that many readers (a) neglected simple solutions in favor of complex ones, (b) neglected obvious solutions in favor of nonobvious ones, and (c) suggested that the correct hints had been put there for deliberately deceptive purposes.

General announcement: I do not lie to my readers. Almost everything in HPMOR is generated by the underlying facts of the story. Sometimes it is generated by humor – I can’t realistically claim that comic timing that precise would occur in a purely natural magical universe. But nothing is there to deliberately fool the readers.

Methods of Rationality is a rationalist story. Your job is to outwit the universe, not the author. If it taught the lesson that the simple solution is always wrong because it is “too obvious”, it would be teaching rather the wrong moral. There are some cases where people have scored additional points by successful literary analysis, e.g. Checkov’s Gun principles. But the author is not your enemy, and the facts aren’t lies.

Now, yes, it is possible that Eliezer Yudkowsky's Author Note on this very chapter is a lie, and he will suddenly reveal a whole series of barriers to paying the debt that will shut off everything from arbitrage to time turners to Dumbledore using the Philosopher's Stone in the manner allowed in canon, without having given us any previous hints as to what they are. But I think Eliezer Yudkowsky is not lying, and that at least one of the many simple solutions proposed (or another simple solution) will work.

If Harrymort regains 'his' former power, he'll have the use of all House Malfoy's wealth. But Lucius still doesn't know what the Dark Lord wants with Hermione Granger.

So if Hermione is vassal of Harry, and Harry is temporary vassal of Lucius, then is Hermione vassal of Lucius?

Or do the muggle medieval rule "the vassal of my vassal is not my vassal" works in magical Brittain also?

But you probably can't do it over the Internet in 1991.

But that is such a vague question. I could go on for hours about entirely irrelevant observations I wouldn't want to get out in public - how I feel about people at work, how much I enjoy certain bodily functions, sexual kinks. Nothing I'd want to tell them, but stuff I would objectively prefer for them to know than that I'd committed a heinous murder.

He has a hundred in his back yard. Worst comes to worst, he arbitrages that up to 60k.

Edit: or time machine+stock market. In retrospect, that's a much better solution.

But he has no way of knowing that. Objects in our world don't come labelled "Chekov's gun," even when it turns out that they should have.

Actually, he couldn't lie - he was interrogated under Veritaserum. That doesn't mean that the topic came up, of course.

Harry Potter is not so clever, part 2. (Perhaps I should call this "advice for Harry," to be less negative.)

"I accept your offer," said Harry's lips, without any hesitation, without any decision having been made; just as if the internal debate had been pretense and illusion, the true controller of the voice having been no part of it. "I should have the whole amount ready by the end of the month." It would take his arbitrage trick, but certainly the Headmaster would let him do that instead of going into debt to Malfoy.

Lucius Malfoy stood motionless, frowning down at Harry. "Who is she to you, then? What is she to you, that you would pay so much to keep her from harm?"

"My friend," the boy said quietly. "As is your son- I would have fought as hard and paid as much to keep him from Azkaban."

"Save it," Harry suggested.

"Let us all go home, indeed." His blue eyes were locked on Harry, as hard as sapphires.

Harry looked further up.

"This is how far I go for my friends, Lord Malfoy. And now that Hermione is safe, I would like your permission to visit Draco. "

Overall: what the heck is Harry's model of Malfoy? Why has he not put any effort into developing it? Why, for the love of wisdom, scare him in public?

It may not be too late to turn him from an enemy to an ally, but Harry is making this too hard on himself. His flair for the dramatic is not helping things, either.