I have received legal notice from my customer, ask me to pay the interst @ 24% even though on of his emplyee signed in my letter head that no dues from my side

Why do you have to pay 'your customer' interest; interest on what ?

You are the customer, not he, right?
We need more information. Did you buy goods, services, what price range, how long has it been?
24% is usury (except credit card companies get away with it) at least in the US.
Why did an employee write no interest due on your letterhead?

If someone is asking you to pay them interest, then they would be a supplier, not a customer.

You really haven't given us enough info to answer you. Try explaining the situation so we can help.