Create an Invoice without a Sales Order. This method allows you to create an invoice directly, skipping the following steps entirely:
If you need to do any of the above steps, most notably creating back-orders or manufacturing items, it is recommended you use the Sales Order Entry method instead.
Warning: Creating a direct invoice can not be reversed! If any mistakes are made you will need to create another invoice, or credit note, to fix any errors.
The steps and requirements for creating an invoice directly are covered under Sales Order Entry. By default, this operation will not allow an invoice to be created if the stock level of any item will become less than 0.
To override the default setting preventing inventory of any item to be less than zero (you will receive a warning), go to: Setup->System and General GL Setup and under the Inventory heading check-mark the box: Allow Negative Inventory.