In Navi Mumbai, @ possession builder asking maintenance amount in cash and refusing to give receipt for it, what to do?

Argument by this builder is that they are partnership firm, and can not open escrow account for maintaining maintenance amounts from members of this project. Further their argument is that it is mentioned in sale agreement that "purchaser will pay maintenance amount before taking possession". Since on receipt of maintenance amount, they will give possession of the flat, so it is understood that buyer has paid the maintenance, so no need for them to give separate receipt for maintenance amount.

My concern is after giving maintenance amount & taking possession, they should not later demand maintenance amount again.