A data medium in the form of a check card is known. Such check cards include a memory in which at least one amount of money is stored. If the user wishes to purchase or use a product or service, the amount of money is debited by the vendor. If the vendor intents to defraud the user, or simply makes an unintentional error, too much money may be debited from the data medium by a vendor terminal without the user noticing immediately that this has occurred. This type of fraud or error may also occur when using other data mediums, including credit cards, debit cards, special purpose cards such as telephone cards and transportation cards, and the like. In order to avoid such fraud or error, further memories are provided in the data medium according to the present invention, wherein one internal memory serves solely for the purpose of accepting amounts of money from outside. The amount needed for a purchase is input by a vendor terminal into a special input memory on the medium. With the aid of a small terminal which the user carries with him, the amount is read and if acceptable, the user uses one key stroke to authorize loading that amount into an externally accessible memory. Since from outside, amounts can only be deducted from this externally accessible memory, it is impossible for erroneous or fraudulent amounts to be deducted from the data medium. Several externally accessible memories can be provided.