Refund of excise duty due to negative price variation

We are manufacturer and supplying goods on Price Variation basis. In a particular instant we are supplying maetrial say at Rs 100 and at time of despatch we know that due to Price Variation Clause, value of material has decreased to Rs 95. But our buyer wants us to issue Excisable Invoice for Rs 100 and then issue a supplementary negative Excisable Invoice (Credit Note) for negative Price Variation in which we should claim refund of excess Excise Duty and Sales Tax. Please confirm if this procedure is legally correct.