Land broker (buyer) has made non registered agreement. (agreement is expired),

1) my brother (seller) is army retired person. He want to sell his property for personal purpose. 2) A land broker (buyer) was made agreement for the above said land on 09.03.2022( 3 months time for execution of sale deed) and given some advance for the agreement. His ( land broker/mediator) intention is to make money by selling the same property to another person during the 3 months time and he( land broker) want to make money by it. Actually we don’t know that the person is a broker at the time of agreement. 3) He (land broker ) made many efforts to sell the said property but all his efforts were in vain. 4) agreement time i.e 3 months time is completed but he is not ready for execution of the sale deed. 5)By the request of the buyer, my brother ( seller) has given extra time for arrangements of money to the buyer ( 4 months extra time is given). But, the buyer has not made any money arrangements. 6) Seller has issued legal notice through lawyer wherein it is mentioned that as per terms and conditions in agreement, 3 months time is expired and hence, the agreement is not valid and the seller is free to sell the property to any outsiders. 7) In reply (from buyer), it is mentioned that we (buyer) are ready with money to made sale deed but the seller has not come forward to execute the sale deed. They (buyer ) has made all false statements in reply to the notice. Now, what we have to do ? . It is experienced that property sale to a land broker is dangerous/ loss of mental peace.