Hi BigBambi ,
Thanks for your kind words.
ATUM works over WooCommerce, and it's WooCommerce that changes the stock following its rules. Please read this article to read how it works. ATUM doesn't change this behaviour by default.
However, ATUM includes a setting to discount the stock only when the Order is marked as Completed (Atum settings -> General -> Change Stock on 'Completed' status). You can read more here.
Please, note that this setting will work as long as your Payment Gateway plugin uses the standard way to change the orders status (I recommend you test it before going online).
Best Regards,
Josá Andrés