Delay Payment

We booked a flat in the month of April 2023 paying an advance payment amounting to 5% of the total value. We received a registration draft which was checked by a lawyer according to RERA rules, and a few points in the registration draft were found to be void and baseless with respect to RERA. The same points were disputed without conclusion until the month of September (end of September). We received a demand draft in the meanwhile (July), and were assured of no delay payment charge until the month of August. By the time the draft was concluded, we got email confirmation stating that no delay payment would be levied on us, and we promptly registered the flat by October mid. Further payments were paid without delay. However, there was a delay payment charge in their demand letters despite the email confirmation, which was again disputed. When asked for a calculation, the request was denied and we were told that they are not bound to disclose the same. Is the demand letter (July) even valid before registration and how should we proceed with the case considering the bullish way the builder is showing?