How Can I Validate Input From User When The Values From Database

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C#

ASP.NET

SQL-Server

i have a sale page where the item and there rate is retrieved from database.
like when i wright a item code it retrieve the item name along with its rate on the text boxes.
now problem is that my user want to change the rate so retrieved from database.
suppose item name is Book and its rate is 100. user want to change the rate from 100 to 101 or 110 0r any value more than 100 or equal to 100.
so my question is this that how can he do it. suppose if he chose 100(which is automatic retrieved from database) or more than 100 he is allowed. and when he chooses or enter value less then 100 an error message get generated.
this will go like this when the value so retrieved from the database will be edited by user at the client side and this will get post back and this post back will do the comparison,if the value is less then the retrieved one then an error script will be displayed. and if the value is equal or more then it will continue the user to enter another credentials.
give me logic or code how to go for it.
i am using VS 2010 and SQL server 2008.