Hi, 
The answer is YES. 

With a simple test in runbot, you can make sure of this by yourself:
1- create product A available in POS
2- make a purchase of 50 and validate it
3- in stock you have 50 on hand
4- Go to POS, scan or select product, quantity= 20
5- Validate payment.

Before closing the session, in a second Tab, go to product form view, You will notice that now you only have 30 on hand and 20 sold ( smart buttons ).

Hope this helps.