How to put validation in sales order?

H Experts,

I have to write a condition on a sales table form in which :

1. I have check is the customer belong to "CFR" group or not 

2. If the customer belongs to CFR group we have to exit but if the customer is not from CFR group we have to check Delivery Term if found then we have to ask for Freight charge.

Please help thanks in advance

Regards