On the main screen, press on the thunder icon, as shown below:
Enter the amount sold. If you want, add a short description.
Press on the green button on the bottom of the screen.
Continue with the sale normally.
Now, select the desired payment method. If you select Cash and enter an amount received greater than the total due, the system will automatically calculate the change.
Congrats, you've just sold a non-inventoried product.