I read this story over 15 years ago. It is probably a lot older. I read it in Serbian magazine (Politikin Zabavnik) but I think that the language of the original is in English. It's a short story, less than two A4 formats long.
The story as I remember it is as follows.
A young man/boy (crash)lands on a desert planet. His only possessions are clothes on his back and a cup. Maybe a gold necklace, too, I am not sure about that. He reaches the biggest city on the planet.
The main trade and commodity on the planet is water. The richest families are those that have wells, people work for water, trade deals are made for water etc. The youth comes to the town and says to one of the smaller traders in water that he is thirsty and would like to buy a cup of water. There might be some flying robots that ensure that all the contracts are obeyed by killing anyone who tries to renege on the deal, I am not sure.
Anyway, the youth is not allowed to drink, not right now. Trader thinks he has nothing of value or something. A bigger water trader, as a joke, offers to sell the youth a cup of water. His fellow traders joke with him, tell him they would loan him a cup or something, because the youth doesn't have anything of value and would have to sell himself into slavery to survive on the planet.
I do not remember what exactly happened, but the end result is that all the biggest traders on the planet sign a contract with the boy. And it goes like this: a boy's necklace for one cupful of water. One cupful, nothing more. The arc words are: What is the value of one cup of water?
The end of story is this: the boy puts a drop of water into the cup. The cup opens up a bit then starts transforming, in fractal designs. It sucks all of the water in the city, maybe even in the whole world and that giant form of cup is floating in the air now, blotting the sun. Some of the traders get worried halfway, try to renege on the deal and are zapped. Once there is no more water and the cup stops filling, the boy laughs at now poor traders and asks them, "What is the value of one cup of water?"