If a mob boss owes you a favor, do you imagine that there is no debt because there is no fixed exchange rate between "a favor" and loaves of bread, or between favors and global production? You could argue that this isn't a debt for whatever reason, but if you ask a bunch of random people, I'm sure you'll find that pretty much everyone considers this to be "debt".*
It is a very good example because again it explains the difference between debt and property !
If I did something for a mob boss, and we wrote out clearly what he's supposed to do as a favor for me, then he has a debt.
For instance: "ok, I will give a false testimony to the police about when you robbed the bank to make you go free-out, but in return, you will go and smash the shop of my competitor, understood ! "
and I give my false testimony to the police, then the mob boss has a debt: he has to smash the shop of my competitor.
On the other hand, if I just propose to give a false testimony, to win his sympathy, with the idea that one day, he will return the favor, then there is no debt. However, I own abstractly some of his sympathy. That could be an illusion on my part. I may have invested wrongly in what I think to be the sympathy of a mob boss, with the idea that one day, I might ask him a favor. But if on that day, he laughs in my face, then I just invested wrongly.
This is btw, something people do all the time. You invest often in doing things for others, with the idea to build up a capital of sympathy. But there are no guarantees of what it is worth. You have simply the ownership of a very abstract entity which is what you think to be sympathy.
You often do that as an employee. You do overtime, you work harder than you are supposed to contract-wise, with the hope of building up a capital of favor with your employer. You can then ask for a raise. But the employer doesn't owe you a raise. He will only give you a raise if he thinks that this will motivate you further to invest your overtime in the company. Not for what you have done. Only for what he thinks he will still get out of you. Your investing in overtime is not a debt of your employer you are engaging. He didn't promise anything.
It would be different if it were like this: "OK, if you can finish this projet 2 months earlier than agreed, you will get a promotion". NOW, there is a debt from the employer to you, if you succeed. If you don't get the promotion, you can rightly complain: "hey, there was an agreement !".
If you did overtime, and then you ask for a raise, without such an agreement, you may get as an answer "I didn't promise anything !".
So in one case, you are the abstract owner of some sympathy capital (you think you have gathered), which may be worth nothing after all ; in the other case there is an agreement, and hence a debt.