Conveyance registration fee

I have purchased a underconstruction flat in Mumbai.95% payment is done and builder has sent offer for possession. He has asked extra amont on name of Betterment of complex and amenities which I have refused to pay till k get a breakup and justification of the same. He has also asked amount for registration towards deed of conveyance.But the same is asked by builder in his account and not in Govt account.Moreover the society formation will take 2-3 yrs since it is a big complex nd part of development is pending. My question is can a builder ask this amount In his account?? I have offererd to pay a post dated cheque since I will not be earning interest on this amount .But builder will till conveyance formalities are started. Secondly; This is a complex of 18 buildings in which 6 in phase one are ready for possession ; another 6 will be ready by year end and next 6 will be launched.Plot is a cluster of small small properties. Another question is would the conveyance be done after complete project is done or conveyance for a single building/group of buildings can be done