Sale of joint property

We have a joint property i.e. a 1200 sq.ft. open plot. There are 6 shareholders of the said property i.e. Someshwar, Manohar, Sharad, Arun, Bhaskar and Usha. 4 shareholders i.e. Manohar, Sharad, Arun & Bhaskar from 6 shareholders has done an agreement of sale on a plane 100 rupees stamp paper with a purchaser for an amount of Rs.5.40 Lacs without any consent of remaining 2 shareholders. The purchaser has given Rs.11000/- cash as a token amount. But, on the said agreement it has been written Rs.20000/- as a token amount from which Rs.11000/- has received by the seller and Rs.9000/- still has to be received. But, on the agreement it has been written that Rs.20000/- has received by the sellers. Remaining 2 shareholders i.e. Someshwar and Usha are not agree with the sale agreement. Moreover, there are 5 legal heirs of one of the remaining 2 shareholders as that shareholder i.e. Someshwar is expired and now Mrs. Kumudini, Anil, Atul, Kishor and Bharti are his legal heirs whose names are registered with land records department. On the said sale agreement 2 shareholders has signed their own and 2 family persons of the 2 shareholders has signed i.e. Narendra has signed on behalf of his father Sharad and Mrs. Suman has signed on behalf of her husband Manohar without any consent or authorization letter of legal shareholders. No power of attorney has been made on the name of any shareholder on behalf of all shareholders. Now, I wish to know whether the family persons of shareholders can sign on the agreement on behalf of the legal shareholder without power of attorney or authorisation or consent letter ? What the law says in this respect ? Further, I wish to know if the remaining shareholders and their legal heirs are not agree with the said sale agreement, can the shareholders sale the said property who has signed the agreement their own ? Further, I wish to know if we cancelled the said sale agreement, can the purchaser take legal action against us, either against all shareholders or against the shareholders who signed the sale agreement ? The purchaser is not ready to cancel the said agreement and to take back his token amount. He is threatening us to take legal action against us. What we should do in this matter to cancel the said agreement without any legal actions, please guide us. Thanks in advance.