Goods purchase including tax & want to sale interstate against c-form

One of my client, based in maharashtra, purchased sanitary material goods inculding VAT, Vat rates depend on material to material some of 12.5% or 5% etc...

When he ws selling the same goods to maharashtra customers, than such customers want the invoice by leying a VAT Seprately... than in such a case he ws reversing the same & than leying the tax & give tax invoice to party.



But Now, he has to sale it to insterstate against c-form, by leving cst @ 2%



My query is How the calculation he should do ?? coz he's doing like  suppose goods of1000 inculding vat

than invoicing ..

                                              1000

les 11.11%:                          111

gross                                    889

add vat 12.5%                     111

total amt                            1000

add cst 2%                           20

total                                    1020



I know this is the double levy of taxation... but please help me out in this regards, HOW WILL BE THE CALCULATION??