The invention relates to a payment system utilizing so called \u201csmart cards\u201d, which include a microprocessor attached to the card with associated memory circuits for storage of transactions, and which via a terminal can be supplied an available and for the card holder useable amount. A fluid loss valve prevents the column of fluid in the tubing from flowing into the lower formation until activated. The other devices may check that the one-way function applied to this input results in the shared output value, and if so, close the communication link.