Buyer not making balance payment

I had entered into an agreement to sell a residential property on 31st September 2019. 10% of the total consideration amount was paid as an earnest money by the purchaser at the time of agreement and it was agreed that balance amount will be paid on or before 31st March 2020 and after that sale deed will be registered. As per the clause in the agreement - 1) Buyer was allowed to develop/construct the property. 2) If buyer fails to pay the balance amount to the seller, then, seller has a right to forfeit the earnest money paid but has to pay the amount spend by the buyer on the development/construction of the property AND If seller fails to execute the sale deed, then, buyer would get the double of the earnest amount paid and the amount spend on the development/construction of the property. Buyer is neither paying the balance money nor giving any satisfactory answer. It's been 2 Months since the end of agreement period i.e; 6 months. My questions - 1) What should I, as a seller, do to make him pay the balance amount? 2) Can an agreement to sell be amended by giving a written legal notice to the buyer AND if the buyer don't reply to the notice within the prescribed time, then, the conditons/terms written in the legal notice be valid ? 3) I am willing to perform my part of the agreement but if buyer fails to fulfill his obligations, then, what should I do ?