WorkGuru's stock sales will let you invoice product lines you don't have in stock.  What it won't let you do is dispatch that sale.  This allows you to invoice a client and then backorder those products to fullfil that order.  WorkGuru has a handy tool which will automate raising the required purchase order.

Create and approve or edit a stock sale to open the stock sale detail page.  WorkGuru will highlight product lines that do not have enough stock on hand to fulfil the order in red.  Note: you do not have the ability to dispatch the order.   If you require payment before raising backorder, you can still invoice as required.

To create a backorder for the out of stock products, click Purchase Stock to raise a draft purchase order. Remove the line items you do not need to purchase and Save and Approve

Issue and receive the new purchase order as per your normal purchasing workflow. Purchase orders raised from a stock sale will display the stock sale reference on the order's detail page. Note: They will not display the sale number in the purchase order dashboard like a project or production purchase order as they are will increase stock on hand when received, not a disbursement.

You can track the status of the back order purchase orders in the stock sale's Related Purchases tab.

Once the purchased products have been receipted into stock, open stock sale from the stock sale dashboard.  The stock sale's line items are no longer red as they are now in stock ready to dispatch.   Depending on the stock sale's invoiced status, click Complete Sale or Partial Dispatch to dispatch the stock sale as required.