Once you begin to place a buy order and input a minimum of CHF 50.-, you will have to complete the process for at least that amount, you cannot cancel it. In case you want to cancel the order before inputting any cash on the ATM, simply touch the "cancel" button.
The machine has no means of returning money. All banknotes accepted by the ATM are converted. It is not possible to cancel the conversion and to get the accepted banknotes back.
Note: to convert cryptocurrencies from one order that you wished to cancel back to fiat, you can simply place a sell order and complete its execution.