I have the same problem. I'm trying to do exactly how Felipe said but nothing happens! If I check Qty by hand I can see in one line what I adjust (like 1 pcs) and in the second line I have at Physical Locations/Stock/Inventory adjustment value "-1" which in SUM is "0". Is this still a bug?
I'm using the latest online version (v12).

You should use inventory adjust. It just works. The other option have something wrong:

It seems to me that updating this value by the product form, has some kind of bug. Used to work in previous builds, but using build 8.0-20160428 does not work, because when you put the new qty available, it makes a move of this quantity, instead of calculating the difference between the previous and the new value. Look at the images... I update to 1000, and I should have 1000 instead of 2000!

Even, if my theory is correct, the move should be of 1000 and it is not..

If you look the moves, it generated a move of 999 instead a move of 1..

I don't know how that function in the product works, but the correct way to do it, is by going to Wharehouse ->Inventory Adjustments, there you create a new adjustment, in the main form select the location and the product(or all products), and in the lines, it will show you the theorical quantity of how much you have of each product on each locations, and the real quantity, you edit the real quantity, and finally validate the inventory adjustment, now your stock is updated!