How to update onhand qty from POS

just i want like

all product type is stockable 

and i want product update from pos screen

when i click on product at that time that product can be reseved for this client 

ex.

product abc, stockable

on hand qty =10

forecast = 10

from pos screen i select  5 qty then abc product 5 was reserve and onhand will be 10 

how is possible

and another 

when i select 1 product at that time 1 product in forecast no more wait and not  more loading required