our sales order created as per our customer requirements after we are dispatched the order. In dispatch order in few item less to customer.

for example:

sales order in 

item 1 is 100 qty

item 2 is 50 qty 

item 3 is 40 qty

our product stock is 

we dispatched order qty below

item 1 is 80 qty

item 2 is 50 qty

item 3 is 20 qty

also item 1 is A vendor

item 3 is B vendor

by the way our vendor are fixed by item wise

how to get a purchase order automatically 

A vendor in item 1 - 20 qty

and other purchase order automatically 

B vendor in item 3 - 20 qty

how do this we lot's time time-consuming to get separate entries and show the back order qty and then create a purchase order