Hi Deepak,
Though I have not heard anybody budgeting in Ariba, probably the best way to do budgeting in Ariba is on receipts instead of orders. In case you have ordered 100 quantity and supplier have only delivered only 60 then you will definitely be receiving only 60 quantities in Ariba and budgeting based on quantities received will make sure that your budget is inline with total quantity received instead of total quantity ordered.
Amit