Reply to a notice of cancellation of order and return of advance

Hi, Please provide any key points while replying a letter wherein the Client is asking to return the advance money paid for manufacturing of the ordered equipment and telling us to cancel the order on the grounds that he not satisfied with the our pricing and he alleged that we have negotiated with his consultant and its a price fraud. He told that he has filed a 420 case against the consultant and is threatening to put a 420 case against us (though it would be false case, as we have got the confirmed order and subsequently payment of advance/s from) We have done partial manufacturing too and incurred cost. Also I want to avoid any future summons (you know it going to another city is always painful), so is there any qualifying statement / disclaimer I can put in the reply letter. Scope of supply: small equipment Delivery: Ex-works Client's City: Raipur Supplier's City and factory is in Bangalore Advance paid is 20% in tune of the Contract value Actual cost incurred is 50% of the Contract Value