Cancellation of agreement

I sold a commercial land to a buyer at a specified amount.it was agreed that the will pay pay 70% of the amount within 4month after which the buyer will be giving the right to start developing the land and pay the remaining 30% within 6 months. The buyer delayed in paying the 70% in 4 months and rather use 6 months to pay but i never complain. After i gave them the right to go on the land to develop it,they came to me that they dont want the land any more because the city authorities claim some portion will be affected by road so they want their money back. I paid a refund of 20% to them and gave them some time to come for the remaining but i couldn't meet my promise. They took me to the police station and reported that i have duped them and i dont want to pay so the police detained me overnight. I agreed with term of settlement with the pollice over a period of 6 months. 3 months after detaining me they have come back to me that they now want the land back. Please can i go ahead and cancel the agreement?