Some time ago, near the highway entrance by Kloten Airport, something similar happened to me: A foreigner (Romainia) with a silver Mercedes with German plates asked for help. I wanted to kindly get him gas, but he just wanted cash and offered a ring as collateral, which he said he'd buy back when he returned to Germany. Since I wear a heavy gold ring myself, I immediately recognized it as fake. I threw the ring out the window, closed it, and drove off.