You're an extremely sweet person, you know that? And full of questions.
Okay. Hypothetically: - YOU have no expectation of return, but I do, and soup when I'm feeling sick is worth lots. So I owe you a big one. - If we agree that we're trading off the bar tab, that's a deal in itself (like a contract) so that's its own thing. I can't pay you back for the soup by paying for the bar tab one extra time.
Everything's weighted, too. With how much that thing is worth to me. Unless you want something that's worth a lot to you, in which case I have to ask for something huge in exchange. Maybe like the soup.