That runs entirely counter to human nature. If you can solve that you may have something workable, but without it "Nonsense" is as good a word as any.
Not so. Tribal societies inherently function on everyone in the tribe working for the overall good rather than their individual gain. Socially, this is how we function by default - people will make sacrifices, extend loans, give gifts, and generally help out those close to them (family, friends, tribe members) without an expectation of something in return.
In our modern world it is often going to be the case that you won't deal with the same people again, or the transaction is completed completely impersonally.
bjelkeman-again has a fair point re: northern Europe; some very good examples to the contrary.
b) Is it really human nature?
This is what social norms are for. Slavery is for now still mostly frowned upon, and in some circles even racism is. Even though using either usually results in huge personnal gains for some.
Only in the places most removed from these incentives do we see slavery. Look at North Korea, much of the population can be seen as slaves (The subsistence farmers for example). The costs of keeping the slaves can be thought of as the cost of running their propaganda machine, arming their military, controlling the border and the loss in relations with the rest of the world. As long the leaders fail to gain a sense of compassion then the only thing that matters to them is their slaves make more than they cost.
Its callous.
Human trafficking is still organized, global, and wildly profitable. There are people in slavery near you, no matter where you are. It's real.
You may also want to look at the few stories of "domestic workers" and "maid" abuse that make the headlines from time to time in your own country (works in any country with a free press).
As said in another comment, slavery is alive and well. It is only frowned upon.