Falk, Lee. Time Is Money

Lee Falk, 'Time Is Money' (1975). Fairly short and to the point, and online. A potential inspiration (idk) for Stephen Tolkin's The Price of Life (1987), which could very well have been an inspiration for Andrew Niccol's In Time (2011).

Compare Kurt Vonnegut’s ‘2 B R 0 2 B’ (1962): not a currency exactly, but an exchange relation. Also compare the Days currency in Terry Pratchett's Strata (1975), and the Oubliette's currency in Hannu Rajaniemi's The Quantum Thief (2011). Also compare real-life LETS currencies and the Economy of Hours, which can in principle use demurrage (naturally dwindling value) to encourage circulation. None of them have yet adopted the idea of killing you if you run out.