My question is, can he really put a case against the property? and block the property, and he is harassing me and telling everyone in the society that he will get me screwed.
It is a false threat posed by the vexatious buyer who out of his grievances is giving this threatening call.
Dont be worried about such gimmicks, ask him to proceed to court, where you can challenge his non-maintainable suit.
Please advise in this scenario what under the law can he do to give me trouble and what shall I do next.
He received the notice today and refusing to return the sale agreement paper and telling he will put many cases against the properties,
You need not worry about the sale agreement held in his possession.
you can issue a public notice in a leading newspaper stating the unregistered sale agreement entered with the said buyer has been automatically cancelled owing to the unwillingness and non-performance to perform his part of contract by the buyer and any attempt made by him to illegally alienate or spreading false information of the property will not be binding on you.
Subsequently you can even file a caveat petition agaisnt the buyer in the jurisdictional court.
This step will protect your interests.