An invoice does not reduce the stock of the article sold. The article is still in stock (TQ00260)

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In order for the stock of the article to be reduced in an invoice, it must be set to "in stock". If this is not the case, no stock is posted by invoices and credit notes.

To check whether an article is "in stock", open the item catalogue, search for the article there and open its detail page.
On the right hand side you will see the property "in stock". If the field is not ticked, you can set the tick by clicking on "Edit" and then save.