Dues until possession

Original completion date of flat as per our sale agreement was Dec 2015. Each building in development is now under a separate RERA registration with ours registered for completion on June 2018. Builder has obtained Completion certificate for the building and conveyanced flats to some buyers without bothering to complete common or internal amenities. We need deductions from final outstanding before conveyancing for interest, amenities not completed/ damaged and property tax, electricity charges,l and society maintenance now due? Is it possible to enforce such deductions or the fight can only be taken up after paying up? The builder is practically all but bankrupt with several other prijects in limbo. So any monies paid now will never be seen back regardless what judgement is achieved later on. Can builder prevent buyer from occupying the property if conveyancing is not completed even if 90-95% of the value of the flat has already been paid, especially given that the deductions claimed will be near or over the sums originally owed? We would like to rent the property out asap. Please advise.