In the short story "And Then There Were None", by Eric Frank Russell (June 1951, Astounding Science Fiction, vol. XLVII, no.4), the "Gands" on the planet use "obs" (obligations) as a form of informal currency, backed only by the mutual willingness of people to honour them. Basically, I do something for you, you do something of approximately equal value for me - or somebody else I owe an ob to. As it's explained in the story, after one of the spaceship's crew has performed a task for a local merchant:

‘Now,’ explained Baines, ‘you’ve done something for me. That means you’ve planted an ob on me. I don’t thank you for what you have done. There’s no need to. All I have to do is get rid of the ob.’


‘Obligation. Why use a long word when a short one is plenty good enough? An obligation is an ob. I shift it this way: Seth Warburton, next door but one, has got half a dozen of my obs saddled on him. So I get rid of mine to you and relieve him of one of his to me by sending you around for a meal.’ He scribbled briefly on a slip of paper. ‘Give him this.’

Harrison stared at it. In casual scrawl it read, ‘Feed this bum.’