Builder-Buyer Agreement-Conveyance Deed of Flat

1st purchaser( original allottee) paid full sale price(say Rs 1.5 Cr ) to Builder & after obtaining possession sold flat to 2nd purchaser (Rs 2.Cr).Then "Deed of Assignment" was signed by Builder and 1st & 2nd purchaser. The flat was sold without executing.C.D.therby 1st purchaser saved Stamp duty of Rs 9 lakhs. The 2nd purchaser was also issued possession of flat and he too without executing C.Dsold flat to 3rd purchaser (say Rs 2.5 Cr.) Now Builder say that he will execute C.D. for Rs 1.5 Cr. only and not for Rs2.5 Cr paid by 3rd purchaser. In that case, it will amount to evasion of stamp duty by erd purchaser of Rs 6 lakhs. Sub- Registrar will intimate purchase value as Rs 1.5 Cr ( C.D value) & (1%zTDS x 2.5 Cr) will be reported to Income Tax Department. These two figures of Rs 1.5 Cr & Rs 2.5 Cr will lead to investigation.I want Builder to execute C.D. for Rs 2.5 Cr. at which I purchased flat from 2nd purchaser who should be necessary party.