Abstract: A method for bidirectional data transfer between a terminal and a smart card with a purse function comprises writing a transfer value from the terminal to a balance-of-account memory of the smart card, writing a write-in control value (E) to a control memory of the smart card together with the writing of the transfer value to the balance-of-account memory, and erasing the write-in control value (E) as soon as the transfer value is completely stored in the balance-of-account memory. In addition, a smart card for bidirectional data transfer between a terminal and a smart card with a purse function comprises at least one balance-of-account memory for storing a transfer value transmitted by the terminal and a control memory. The balance-of-account memory is assigned to the control memory for storing a write-in control value (E), which is alterable depending on the presence of a fault.