Dear Bijay,
Condition is already open for price and quantity change. And proposal which Nilesh gave me almost covers requirements. Only problem is that if I do what Nilesh propose, I can change price even with existing limit if I try to change in Item overview section in the field Net price. If user try to change price in this field and not in pricing elements under Condition tab, system will allow this change (even limit is entered in contract condition). And it should not happen.
Regards,
Harry