Penalty for late payment

Hi, i have bought a resale flat in a housing society in Thane, Maharashtra. The sale agreement was registered on April 1, 2021. Now only the bank loan contribution is pending. The bank has raised queries regarding the development agreement done by builder in 2006 and 2007 . These lease deeds were not duly registered so bank has asked for secon legal opinion. Otherwise the building has occupancy certificate, registered society and conveyance process is underway. Due to this query, the loan sanction process is delayed. The owner is demanding penalty interest from me at 18% interest saying that other banks had not raised this issue and disbursed loan for other flats in society. The sale agreement mentions that the balance consideration (bank loan) to be paid within 15 days from registration and receipt of all original documents and related papers from the owner. The owner has given me all the original documents. But he said the society is not having the documents (lease deeds in 2006, 2007) asked by bank. Also, i had obtained loan sanction letter from same bank based on my salary in Feb 2021 itself. The delay is only because of property document issue. Also, the agreement has no mention of any penalty interest in case of late payment. In such situation, am i liable to pay penalty interest as demanded by owner? What should be my course of action now?