Olympia Infratech Pvt Ltd - Penal Interest for no fault of buyer

I booked an apartment with Olympia Infratech Pvt Ltd Chennai in their Chennai Navalur Olympia Opaline Sequel under construction project in ****. I paid the initial **% of total amount as it was required to enter into sale and construction agreement. I was seeking clarifications on Sales & Construction Agreement and informed them at that time, further payment will be made only after satisfactory resolution of all points and registration of flat. it took over * months for Olympia to resolve the points raised and register my flat as per Tamilnadu government requirement. Subsequent to registration, I cleared dues and made all subsequent stage payment as per project completion. Olympia didn’t ask for any interest when I re-started making payment as soon as registration was done. On the final instalment and at that time of handover, I received a demand notice from Olympia with a penal interest demand of Rs *.* lakhs and when I raised it with them, there is no proper response. Olympia is demanding to make these payment for handover of the flat. I provided them evidence that the delay in payment is due to Olympia and I am ready to make interest payment if they can provide proof that I had delayed any payment. It's going on for last * months without any resolution. Having invested all my savings in this property, I have lost out rent for the last * months plus mental agony & uncertainty. I request your help in resolution of this issue.
Thanking you,
Karthik Anbazhagan