sales order enhancement

When I create a sales order in VA01, when I enter less than 3 different brand( material in line item), The system should first generate message, after that I should be able to block that order with giving proper reason and then I should be able to save that sales order.

so that the higher authority can check it for further processing.

This is the scenario.

Can anybody suggest me the exact user exit name with simple coding.

naresh