A computer implemented method of performing a transaction using a payment account, typically handled by a payment account manager (PAM), comprises the steps of: receiving a first message from a top up terminal indicating that funds have been transferred to the payment account; setting or adjusting a funds limit of the payment account based on information in the first message; receiving an authorization or pre-authorization request from a first terminal and commencing a funds aggregation; receiving a second message indicating a pre-defined amount of funds which may be used in the transaction; blocking an amount of the funds limit of the payment account equivalent to the pre-defined amount; and sending an authorization response to the first terminal.