Builder asking for interest for Late payment which was not mentioned during booking.

I have booked and registered a flat in a complex of 7 Tower 500 flats from a reputed builder (under RERA) which will be completed on Oct 2022 During booking they have mentioned that they will be flexible regarding payments. On Jan 2020 I have booked a flat price ( 38 Lakh)with 4 Lakh on initial which was at level 6 and told me to keep making payment till the completion of the project but after few weeks they send me a demand of 16 Lakh as they mentioned that the tower is, on completion of 9 floor. till Sept I managed to pay total of 10 Lakh but now again I have received another demand of total due 21 lakh as it has completed 14th flr. Builder is now asking me to pay interest from the first day of the booking till the last demand sent to me as he has mentioned the clause of interest on late payment but during the time of booking they were flexible with everything and have never mentioned about the current payment demand to pay they only asked me the 10% during booking. The builder is only completing 3 towers and the other towers are still on the 1 and 2 level. Other facilities are not yet constructed. I was not aware of this type of construction process. The builder still have 300 flats to sell and price is still the same when I brought on Jan. They have mislead me just to get the booking they should have mention me that I will have to pay till the 6th flr which was 10 lakh at the time of booking then I shouldnt have booked that tower I could hv opt for other towers which are still on level 1 and 2. Due to pandemic I am unable to get loan. Please everyone suggest me and help me with your advice.