You have not stated that whether this agreement of sale was entered between you both by a registered or an unregistered document.
If it was by a registered document then you may have to approach civil court with a suit for cancellation of the registered sale agreement for the reasons you may rely upon.
If it was by an unregistered sale deed then you can issue a legal notice communicating your decision to cancel the same for the reasons you may rely upon and can intimate your ready and willingness to return the amount received as advance towards sale consideration in a time, date and place convenient to the buyer upon cancellation.
You can also plan to issue a public notice in this regard intimating the decision of cancellation of sale agreement in a local newspaper which may protect your future interests in case the buyer approaches court seeking any relief in this connection.