Builder agreement breach, want to cancel it.

The said property is in my mother's name which she inherited from her mother under registered will. We are Christians. We entered into a registered agreement for constructing a multistorey building on 60:40 ratio. This was done in 2014. Later we realized the size of land is too small for this and legally not viable. Now the builder has done nothing since then. They are demanding an exorbitant amount to cancel it which we are unable. We want to sell it out now. How to get the agreement cancelled as soon as possible?