How to cancel a sale agreement if the other party does not want to

My brother has taken 1 lakh advance and signed a ikrarnama on 500 stamp duty for our father's property. I haven't signed it and neither has my sister. It states that we are selling the property and will register it once they pay the remaining money on a later date. It has a clause that states that we have given the possession to the buyer and they are free to do whatever they want with the property. Moreover, it says that property is free from any dispute but it is in fact disputed according to old family agreement and part of our property is under our cousin's possession. We cannot register the property and now are worried that they might take forceful possession. They are also not willing to give all the money in cash. We wish to cancel agreement but they are not willing.