1. first thing - possession can only be offered when the OC is in place. So the builder cannot handover possession without an OC. It is plain illegal
2. as the builder is not agreeable to your offer, ask him to share with you the construction linked payments which are provided to other buyers who pay the builder in installments based on the stage of construction
3. you can then ask the builder to apply the same construction linked payment for you as well
4. it is better if you could get the certified copy of a sale agreement from the registrar's office [the latest agreement which is registered by the builder] to know the construction linked payment offered to other buyer who are paying in installments.
5. the builder has given you a timeline of upto 6 months for registration and possession. However i would never believe that due to the lockdown. He may be pressed for funds and would take more than 6 months to complete the project
6. also if you pay a substantial sum of money before OC, you will have to pay GST at 5%. However if substantial money is paid upon or after receipt of OC, GST will not be payable. You need to factor that in
7. if you think that the apartment is a good deal and you dont want to lose on it and also if the builder is insisting for 90% payment then insist for registration. The builder cannot delay registration of sale agreement. As per RERA the builder is mandated to register the agreement on receipt of 10% of consideration. In your case the builder would be receiving 90%. So how on earth can he even tell that pay 90% now and registration will be done later? its plain illegal
8. also if you wait till the OC is received and apartment is ready to move, the builder may not offer the same rate as he is offering you now. Finished apartments are generally expensive than unfinished ones. So thats a commercial decision you have to make for which lawyers cannot advise