On forfeiture of advance amount in an agreement to sale

On 19th Feb, I signed an "Agreement of Sale", on Rs 20 Non Judicial Stamp Paper in Hyderabad, with a person for sale of house for 90 lakhs and Rs 10 lakhs was paid to me towards advance and balance payable at the time of registration of sale deed. The important terms of agreement are a follows. 1. The purchaser shall pay the balance amount of sale consideration within 90 days 2.If the vendor fails to execute the sale deed in favour of the purchaser or his nominee on mutually agreed date, the vendor shall be liable for the specific performance as per law, 3.If the Vendor fails to execute the sale deed in favour of the Purchaser or his nominee or nominees, shall pay BACK the advance money TO THE EXTENT OF 500000 WITHOUT ANY DISPUTE OF LAW. Now in spite of several reminders and followup the buyer is just postponing to complete the purchase obligation. my question is 1) wheather i can forfeit the amount of advance on cancellation, on the ground of contract and loss of rent 2) can i sue them for damages 3) what is the procedure for cancellation of agreement to sale