Hi Vinit,
First of all, I am not aware of "group condition style" rounding functionality in SD.
IMHO Arijeet's proposal for validation (could be an error or a warning) looks good to me.
If you don't want a development, you might want to use a manual check.
You might want to set-up a delivery block at the document header.
In customizing of the sales document type (VOV8) you could set the delivery block 90 (internet sales)
so the entire sales order is blocked for delivery automatically.
Then a administrative user should enter the sales order and release it if it conforms to the rounding requirements.
Of course this is not the solution you want, but if your boss does not want any development, then it has to be a manual solution (if the functionality is not available in the standard system).
IMHO maybe it is not a good idea to "hard code" these kind of commercial requirements, because they can easily change. Maybe in the future you company just decides to drop the rounding requirement altogether and ships any quantity if the the sales order is over a specific amount.
Another solution would be to validate this kind of specific requirements in a special "CRM Telemarketing" screen - maybe built on .Net with SAP Connector? The specific validation could be done in the .Net application and the validated Sales Order would go to SAP (Sales Execution) through the typical BAPI for Sales Order upload.
Best Regards,
Franck