Firstly there is a liability of service tax/ GST under this circumstances because in case of JDA agreement since the consideration i.e. a share in land is provided beforehand i.e. Before completion on flat liability of service tax / GST arises.
Liability of GST doesn't arise in situation where you pay the consideration after the completion of flat.
So the demand of his service tax/ GST is correct.
But you must check the amount he is demanding because the amount of GST is 12% of the consideration and now he will collect GST and not service tax and I don't think he can collect late fee from you because it was his liability to collect and pay service tax / GST in time and not yours so ask him that you are ready to pay gst amount but not late fee amount.
If he is not providing you possession even after this then you can file a case against him keeping the JDA agreement as base.
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