Thank you. I now have a better understanding. Now I need to try & discover how to trigger the stock movement from a sales invoice.

It may be better to see if you can adapt to the way Odoo works. The Sales Order represents the agreement you have with a Customer to buy items from you (so does not affect stock until you fulfill this agreement - or not). Similarly, a Purchase Order represents the agreement you have with a Vendor to receive items from them (so does not affect stock until they fulfull this agreement - or not). If what you want is a single step that affects stock, you should work with a Delivery Order (customer side) or Receipt/Incoming Shipment (vendor side). As Ermin has already said, these documents don't involve money, just record the movement (increase or decreased) in stock valuation. Sales Orders and Purchase Orders don't affect income or liabilities either - as nothing happens until the agreements are fulfilled (or not).