This is a current Nebula nominee which first appeared in Asimov's. The pdf for the story can be found here.
Lanchi Ronpa, grandson of an English expat, has lived in the Incan Empire. He is called on to translate between an American and the god-emperor himself. The American claims to ave a cure for small pox and is willing to give it in exchange for 4,000 times his weight in gold, the weight of the sunrise. What he doesn't know is that the Incans don't see gold as money but as spiritual. They will have to sacrifice 1200 to appease the gods for this trade.
Loddington's goal is political and multi-pronged. He wants money to fund the American independence war against Britain. He wants to create peace between the Incan empire and the newly independent American colonies, and finally, he wants equality, a brotherhood of men. But the irony is that he carries with him a slave who is his half-brother but does not consider the boy an equal.
Negotiations are difficult, and the first pox trials succeed. They sign, but little else will go according to plan, including a revolution fomenting within the Incan Empire, not to mention other double-crossers who want to ensure the safety of their family and other citizens.
This well illustrates how humans often claim to hold one value though we truly do not--at least not in all cases.