Abstract: The invention concerns a control unit (12), a control system (10) and a method for the checking of transactions carried out in a cash register (11). The control unit (12) has interfaces (27, 28) for connection to at least one cash register (11) and at least one receipt writer (13), and it is characterised in that it comprises at least one regulatory unit (21) in which encryption algorithms (22) are stored, which encryption algorithms (22) are used to generate a unique code based on transaction data (24) from the cash register (11), which unique code is sent to the receipt writer (13) to be printed onto the receipt. The authenticity of all receipts that are generated by the system can in this way be subsequently checked.