We are manufaturing the Vehicle Tracking boxes and we Registered under Tamilnadu VAT. Our Head office which is functioning in Gurgaon, We send some boxes to our head office and there they made some sales but our Head office is not registered under their local sales tax. The sales made in our head office (Invoice number providing from Chennai office) is considered as the sales made from our Chennai office and it shows in our CST Return as interstate sales this method of invoicing is Correct? or Our head office need to be registered under their local sales tax for the sales made there?. Kindly advice