1. As per the terms of the said sale agreement , ' time is the essence of the contract'.
2. In that context you can proceed to cancel the sale agreement in the event the purchaser fails to purchase the property within the stipulated time.
3. Is further appears that on expiry of the stipulated 3 months time you have the purchaser several reminder but he failed to complete the sale.
4. In that circumstances send him a a letter cancelling the said agreement along with the refund of advance money and then enter into a sale agreement with another buyer on such terms as you mutually agree afresh.