Can I cancel an unregistered sale agreement of a land?

We had a disputed land which was under a court case. In 2009, we entered into an agreement on stamp paper with a buyer with the following conditions. (1)We accepted to receive the amount of Rs. 100,000 from the buyer. (2)A rate was decided to sell the land to the buyer at a fixed rate once the property comes on our name, if we win the case. (3)The agreement was made on a stamp paper and attested by notary but it was not registered. (4)Out of the 8 owners, 6 owners signed the agreement but 2 did not. Also, one of the owners who signed the contract has passed away. (5)There was no expiry date on the agreement according to which the agreement could have automatically stand cancelled. (6)There were no witnesses who signed the contract. Since one owner has passed away, the remaining seven owners now want to enter into an agreement with a different buyer at a different property rate though the case is still going on. Can we cancel the first agreement even if the first buyer does not agree, stating that the first agreement was invalid as all the owners did not sign the agreement? We are still ready to give him Rs. 100,000 with interest.