Abstract: A method, a reverse vending machine, a data processing unit and a computer programme product for initiating a data transaction in relation to a self-service purchase, including the splitting of or the making of a supplementary payment to a deposit credit. A first data is generated, which is representative of a first value corresponding to a deposit credit of an amount of packaging items of a user. A purchase transaction is initiated on the basis of a second data, which second data represents a second value of the purchase in question. A third data is generated if the second value is smaller than the first value, and a pay-out transaction is initiated in the form of a credit to be paid out to the user or a further purchase transaction. A fourth data is generated and a supplementary payment is started.