I would like to add something like this if it's possible.

Two buttons - I know, there is update option and PoS - but I would like to have just One button to take 1 unit off or add 1 unit to stock without using all those advanced mechanisms.

Buttons as a buttons is not a problem but action ....

Can you help me with it guys ?

 Or maybe with input field in case if you want add/remove more units.