If you sell an item that you don't have in stock from POS (Point of sale), the item will show up in a negative number on your display.
After you click on 'total', before you print the receipt, in the 'status' select 'to order'.
The system will display you a warning that you don't have enough quantity, from here, just proceed as a sales order.
This transaction will go into your sales order list, and you will see that the quantity status is 'No quantity' and that the status of payment is 'advance payment'.
From here, do the fulfillment as usual and generate a purchase order. Don't forget to change the warehouse if needed!
After you receive the items, the status of your POS sales order will be 'available quantity'.
From here, you will pack this POS sales order, in order for items to be deducted from your inventory. You can invoice the document as well, in order to close the full circle. After you invoice it, the payment status will switch from 'advance payment' to 'paid'.