land encroachment by dalits

My mother had purchased a plot in of the rural areas of Bihar. Now after 20 years of the purchase some dalits have encroached a part of it and claim that it is their ancestral property. We tried several ways by going to the police and producing the original documents but things are not getting any better. Since Bihar has its own state laws for Dalits so any kind of stringent actions from our side can create big issues for our family staying there. Kindly let me know what can be done from the legal aspect and how useful can it be?