User exit for VA01 - No range for sales document type

Dear Experts,

In our project, we have 8 sales organizations and these are assigned to 8 different company codes. Actually our client doesnt want to maintain different document types for each Sales Organization. We need to maintain one document type for all the sales organizations and different No ranges for each sales organization. So can any one guide me on User exit available for this purpose.

Thanks & Regards,

Rahul