Do friendship and money mix?

What's the difference between a friendship, or any human relationship, and an economic relationship? (for the transcript of this discussion, go here.)

In our discussion of Emotional Labor, from February, we talked about the intersection between emotional relationships and economic relationships, and it seemed that everyone drew a line at some point between a friendship and an economic relationship. Different people placed that line in different places, but there was always a line. So today I want to discuss a possible reason for why this line exists. I also wonder if the rationale for the line is the same, even if the conclusions about where exactly to draw it are different.

Let's use an example brought up during the emotional labor discussion. One person said that they wished that they could ask a friend to take them to the airport and just pay them 500NT instead of owing them a favor.

What are the arguments for this action?
What are the arguments agains this action?
I'm not asking because I want to start a fight in our discussion group! I just want to see more clearly the arguments on either side of that line between friendship and economic relationship, before we go on to discuss the next thing.

What is the difference between an 'obligation' and a 'debt'?
"On one level the difference between an obligation and a debt is simple and obvious. A debt is the obligation to pay a certain sum of money. As a result, a debt, unlike any other form of obligation, can be precisely quantified. This allows debts to become simple, cold and impersonal--which, in turn allows them to be transferable.*

If one owes a favor, or one's life, to another human being, it is owed to that person specifically. But if one owes forty thousand dollars at 12-percent interest, it doesn't really matter who the creditor is: neither does either of the two parties have to think much about what the other party needs, wants, is capable of doing--as they certainly would if what was owed was a favor, or respect, or gratitude. One does not need to calculate the human effects; one need only calculate principal, balances, penalties, and rates of interest. If you end up having to abandon your home and wander in other provinces, if your daughter ends up in a mining camp working as a prostitute, well, that's unfortunate, but incidental to the creditor. Money is money, and a deal's a deal. Debt, p.13-14

Do you agree or disagree with this statement: "Surely one has to pay one's debts!"
Let's ask this again: What's the difference between a debt and an obligation to a friend?

*You know, in The Mystery of Capital, the author argues that it's the impersonal transferability of property that makes Western capitalism such a powerful economic engine. So it seems quite possible that our entire economy and therefore civilization is based on the impersonal transferability of debt. (Here's a summary of the book.)

Debt is enforceable because of violence
From this perspective, the crucial factor … is money's capacity to turn morality into a matter of impersonal artihmetic--and by doing so, to justify things that would otherwise seem outrageous or obscene. [It may not actually only be about violence:] the difference between a "debt" and a mere moral obligation is not [just] the presence or absence of men with weapons who can enforce that obligation by seizing the debtor's possesions or threatening to break his legs. It is [also] that a creditor has the means to specify, numerically, exactly how much the debtor owes.

[V]iolence, or the threat of violence, turns human relations into mathematics … .[It] lies underneath the essential fabric of our institutions today--state and market, our most basic conceptions of the nature of freedom, morality, sociality--all of which have been shaped by a history of war, conquest, and slavery in ways we're no longer capable of even percieving because we can no longer imagine things any other way." Debt, p.13-14

Do you think the institutions of government and market are based on the threat of violence?
Do you think that it's possible to have peaceful human relations without the threat of violence? Do you think human relations can ever be completely free of the threat of human violence? Individual relations? Group relations? Humans as a whole species?