Builder demanding huge interest as delayed payment

I am purchasing apartment in Hyderabad. Agreement dated 10/04/2021. It was an under construction project. Payment terms as per agreement was stage wise. In first quarter of 2023 builder builder asked for the part payment claiming the flooring was completed till that time 90% of the agreement amount was already done from my side. But the work was not properly finished i held the amount asking them to finish the work properly. They were doing lot of corrections after i demand to do rectification. Since then till now the works are done and i also paid some parts of the due amounts out of the remaining 10% during this period. Now the property is ready and i paid almost full amount and some last fund is pending from bank loan which supposed to be paid during the registration. In Sept 2024 they given me a payment remainder demanding the last amount and stating that if i am doing late they will charge penal interest. But there was no penal interest amount mentioned. The exact statement was - "Pay the due amount with in 3 days to avoid any penal interest". I visited apartment to check the work & found lot of errors & requested builder to rectify. On Sept 2024 to Nov 2024 the final corrections & rectification works happened in the apartment. Now while i am asking for registration they are asking for penal interest as payment delayed and saying, without clearing the penal interest they are not going to come for registration. Even on 11 Nov 2024 they given me a demand letter for final loan amount release, there was no demand of penal interest. Still there are minor corrections & rectifications are pending they are working on those, parallelly i am asking for registration and handover. Should i pay penal interest in this case? Whereas till Sept 2024 remainder letter & Nov 2024 demand letter there was no mention of penal interest amount.