Products to decrease in quantity on order status change

[Astarta]

Hello everyone!
I need the products from orders to decrease in quantity after I apply certain statuses to them: for example, Delivered, Reserved or Sent. It seemes completely logical that Prestashop could do that out of the box but then I tried and it didn't.
Also, I need them to "return back" if the order is cancelled.
I sell clothes. So, for example, there was placed an order on a blouse (1 item of it) and I have 1 item in stock. I see new order, call the customer and arrange all the details about delivery. After that I change status from Preparation to Reserved. I would like the quantity of the blouse to become 0, so no one can buy it. Or, I can even deliver or send it very quickly so there is no need to reserve. I change status from Preparation to Sent or Delivered and I, again, expect the blouse to be 0 in stock after that.
And, alternately, if the client cancelled that order and it would not be delivered, I change its status to "Cancelled" and I expect that the blouse's quantity is 1 in stock again.
Please, answer my question if you know: is it possible to do that with prestashop settings or should I add some code? Could this be added without spilling too much blood on my keyboard after fighting with the MySQL and code stuff for many hours?
P.S. What does the option "Consider the associated order as validated" mean when I edit order status settings?

[metalines]

Hi, 

 

Did you solve this?