Regarding the partition of my father's self acquired property

We've decided to purchase a site and entered into a sale agreement with a Seller on 200/- Stamp paper(un-registered, agreement date 15-Sep-2018,agreement period is one month). I've paid an advance amount of one lakh and two thousand rupees. Surprisingly, during the last week of agreement period, Seller sent a SMS to my mobile number stating they are not willing to sell the site and asked us to take the advance back. When called and inquired the reason, they said that they are willing to sell the property to their brother at more price. Hence I've issued a legal notice, a day before the end of agreement period, stating I have arranged the funds and requested them to come and register the said property. They sent a reply to the notice, stating they are not willing to sell the property due to some family issues and asked us to take the given advance amount. We wanted to meet them for taking advance amount, they went on postponing after 2 months they turned up and said they will give me only the amount I paid and have refused for giving me any damages I have undergone. They have threatened us as its un-registered sale agreement and the agreement period has ended I cant do any claims on this and they can sell the site to a third party. Please guide me how can I proceed further. If I file a legal suit can I get the site registered from them or can I get my advance amount back along with the damages I've incurred.