Interest charged by Builder

Dear Sir's, Need guidance. I had booked an underconstruction. flat in Apna Ghar Phase 3, Mira Road on 30 June 2018 by paying 10% money. On 22 Sept 18, I got a first call from builder to do stamp duty registration which I said to proceed after Pitrapaksh. Agreement was registered on 24 Oct 18 and I have rec'd first demand letter on 30 Oct 2018 demanding another 25% payment along with Interest charged @ rate of 25% i.e 14000 Rs. I disputed with builder how you can charge me Interest since I have rec'd first demand letter on 30 Oct 18 only. But he is saying my slab got ready on June itself, hence charging. Builder had not sent any demand letter before. Pls advise is he right in this and what can I do to sort this issue.