It's possible to transfer an incomplete cash transaction for billing if the transaction is not paid in cash or card.
You can start the transfer by clicking To Billing -button next to the Card Payment -button at the checkout.
The software will then ask for a confirmation of the cash transaction transfer to billing. Click the Save invoice -button to continue.
The invoice will be saved as an open invoice in the Billing section of the software. Click the Show invoice -button to move to view and edit the invoice. The invoice will be open to the new tab. You can continue using the checkout normally by clicking the Close -button.