Documents copies for vacant plot given to prospective buyer

Hello. My father owns a society plot in Hyderabad We wanted to sell it and my mother new a broker who then introduced us to his friend(another agent). This agent informed us that he knew a party who are interested in purchasing the property (a plot). As the price was ok we decided to go ahead with the deal and we met the "prospective buyer" 2 times. They had asked for copy of the docments like sale deed copy, encumberance certificate and link documents. The first time we met the prospective buyer , a male who then introduced us to his business partner, a lady andinformed us she would be making the purchase in the company's name. Eversince we had given the copies of the documents (sale deed , ec, link documents) we have not heard from them. I am not concerned about the deal. Both the agents and the so called prospective buyer are not replying and despite numerous requests we are unable to get back the documents. Even if we did get back the documents they must have already made copies. My question is 1) can these people misutilise these documents to sell this plot which is in my dad's name? 2)What course of action to take in order to safeguard/protect the plot and to make sure that this party cannot cheat us. 3) Will approaching the society and giving them a letter in writing by the owner (my dad) be useful? Please let me know so I can take the necessary action Thanks .