A method and system for operating a new financial instrument and payment card integrates savings and credit transactions belonging to the same payment plan into a single account. Transactions can be carried out at different periods in time, for any amount, agreed on with different fixed or adjustable interest rates, without losing the payment conditions of each savings and credit. A financial account card as a financial instrument can be used to carry out long, medium and short-term savings and credit financial transactions. The card can also be used as a payment card to buy and sell anything of value. The card is operated through financial institutions and service centers by a computer system, which manages transactions between cardholders themselves and with financial institutions, maintains account balances, determines installments for debt and savings balances, manages available credit limits and guarantees, and manages other financial and payment services.