You could claim that since money isn't a claim on a specific thing, or from a specific person, that it doesn't qualify as debt. But this is just a matter of definition, and not a universal one because plenty of people would draw that line in a slightly different place. You can certainly choose to define debt in a narrow sense that excludes various categories of similar concepts, but what good does it do you?
If you have a hammer, that is not debt. If you have bread, that is not debt. If you have a car, that is not a debt. If you have a knife, that is not debt. If you own a book, that is not debt. If you have a can of milk, that is not debt. If you have wood, that is not debt. If you own petrol, that is not debt. It is *ownership of an asset*.
Monetary assets, whether they also have a direct use (dollar bills to burn them, gold to make juwelry or to use industrially, like petrol, wood, milk...) or not (bitcoins), are just assets you OWN. They are not debt. Nobody can be forced to give you anything, or work for you, because you are the owner of a piece of wood, a piece of gold, a bitcoin, or some milk. Nobody is liable to anything to you because you own a car.
However, if you have a debt asset (if you have lend your car to someone, and you have a piece of paper where that person engages himself to give you that car back, plus some petrol), then that person is liable, has a debt, and if you OWN that debt, then that asset can also be "money". So debt is a specific kind of asset, and yes, it can ALSO be money.
If you have a government bond, then the government owes you stuff, and that bond, which is a debt of the government, can also be money. It is an asset which is a debt, and being an asset, it can be traded for other stuff, and hence become money.
But money is not debt per se, and most of the time, it isn't.
By recognizing money as an abstract form debt, you are able to quickly purge from your mind all sorts of silliness. It is a more useful way of thinking about things.
You should read Rothbard:
http://mises.org/sites/default/files/What%20Has%20Government%20Done%20to%20Our%20Money_3.pdfNow in the broadest sense, an "asset" is merely something useful, which can be "useful for use" or "useful for exchange". This obviously includes goods, money, in any form, and debt, again, in any form. Saying that money is an asset isn't an argument for or against including it in the category of debt.
No, debt means that somebody is liable. That he has engaged himself in doing something or giving something. Otherwise the word "debt" only means "property" or "asset".
If you have a debt, then you are not free to do as you please.
If you are holding a debt liability of someone else, then that person is supposed to act, and can be enforced to do so, unless the debt is really beyond his capability.
If you took a loan at a bank, then you are not free to decide not to reimburse it. It can be that you are broke. But the bank will first take all you have (or most of what you have). You have no choice. THAT is a debt.
If I have gold, then that gold is not a liability to anyone. Nobody can be FORCED to do anything for that gold. Most people will CHOOSE to do so. But they are free. They could also choose to do something to get my car. But my car is not a liability either.