Maintain Inventory Current Stock

How to maintain inventory for multiuser system using same inventory data. eg. i have 5000 quantity of goods for sale.
one user loged in and saw available quantity is 5000. now he prepare an invoice of 3500 quantity. During this transaction another user loged in and find available quantity
is 5000 and he create an invoice of 3000 quantity when the invoice process is complete stock is not available. how to maintain same situation for so many types of product and
in an invoice. for each type of product have same type of problem.