Regarding cancellation of agreement

We made an agreement for sale of our house on 13 feb 2017 and agreement tenure is for 6 mnths.they paid an advance of 3lakh rupees.after 15days we approached him and asked for payment of certain amount in april month,he agreed that he will pay the amount in that month but we did not mention abt this payment in april in agreement.he did not paid that money and the registration is 13th august.now we would like to cancel that agreement and we will refund the which they have paid in advance.is it possible to cancel our agreement??moreover it is under gramapanchayat (notary land)