1. If project is registered with RERA, then builder can sell parking space except open car park
2. the sanctioned plans reflecting the position of car parks would have been obtained by builder. So I do not see any reason why the builder is hesitant to mention the car park identification number and position
3. as car park (stilt or covered) can be sold then the same can be stated in a separate agreement which can be registered. The builder's reason for issuing an allotment letter cannot be understood
4. ask the builder to show where on the sanctioned plans the car parks have been earmarked. If builder is not agreeing for a separate agreement that may mean that car parks are not sanctioned in layout in first place and the builder wants to hide that
5. or it may be that by entering into a separate registered agreement the builder would become liable to pay tax on the money you would pay for the car park, which he wants to avoid. And therefore insisting for selling car park on allotment letter by accepting cash from you. Dont agree for the latter