I guess the analogy still works because not all debt is monetary. You might owe a debt to the mob. Perhaps they help you out of a sticky situation but let you

I guess the analogy still works because not all debt is monetary. You might owe a debt to the mob. Perhaps they help you out of a sticky situation but let you know that some unspecified favor will be required in the future. So you have a debt, but what exactly you owe is loosely-defined and might cause great chaos in your life at some inconvenient point when it's time to repay.