What are my options if the buyer breaches the sale agreement?

We made a registered sale agreement with a person (buyer) to sell our scheduled property in September 2020. The agreement states that transaction has to be completed within 5 months of signing the Sale agreement. That is remaining amount to be paid and Sale deed executed before March 2021. The sale agreement included INR 50 Lakhs payment to us initially. However, the buyer has breached the contract and asked us to sell the property for below market price. It has been 4 years since the Sale agreement. The buyer now wants us to return the money taken or he has threatened to take us to court for breach of contract. There is no forfeiture clause in favor of seller(us) with respect to the money taken from the buyer. Can any construction take place on the property? Is there a possibility of buyer getting a stay or injunction against construction? If he takes us to court what are our options? How can we prove he is the one who breached the contract?