You have stated to have issued a legal notice to the buyer but have not stated that whether if the sale agreement was done by a registered deed or it was an unregistered document.
If it was executed by an unregistered document, then you can communicate your decision to cancel the sale agreement owing to the failure of the buyer to perform his part of the contract despite giving him sufficient time.
Subsequently he can issue a public notice through any local newspaper stating that the said agreement of sale of the property in question has been cancelled owing to the failure on the part of the buyer to perform his part of contract.
This will confirm the cancellation of the sale agreement.
If sale agreement was executed by a registered deed,, after the expiration of the period or time granted in the legal notice to the buyer, you may have to file a suit for declaring the sale agreement as cancelled with a direction to the sub-registrar to register the cacnellation deed without the presence of the buyer since the buyer is not willing or not cooperating to cancel the sale agreement.
In the pleadings you may have to mention that the buyer is neither ready and willing to perform his part of contract nor he is cooperating to cancel the sale agreement by executing a cancellation deed before sub-registrar, hence the court to declare this sale agreement as cancelled for the reasons mentioned therein.
Without taking legal actions, it may not be possible for you to solve the issue in the absence of any cooperation from the buyer.