TDS is applicable on purchase of flat only if both the following conditions are satisfied:
1. The purchase price is Rs. 50 Lakhs or more
2. The seller is a resident in India
So, first check whether TDS is applicable. If TDS is not applicable, there is no cause of action at all.
If TDS is applicable, the demand may comprise of TDS amount, Interest u/s 201(1A) and Late fee u/s 234E.
However, since you have already paid the full purchase price of the Property to the Seller, you can claim the benefit as per First Proviso to Section 201(1A) subject to the conditions mentioned therein. This will waive off your TDS and Interest liability. Also, if the TDS liability is waived off, then there is no need to File TDS Return which in turn will nullify the Late fee u/s 234E.
Request you to send all the Transaction details to my Email ID firstname.lastname@example.org so that I can guide you better in this regard.