20 rs stamp paper without owner sign

Respected sir/Madam Exactly 12 years back my father bought some hand loan by keeping land papers as surety . Actual owner of the land is my mother. 2 months back my father expired. And the party who gave money is claiming that my father sold the property to them and they are showing a 20 rs Bond paper as proof. And when I read the Bond paper the agreement was written as I am selling this property , and my mother was not involved in this agreement and they created some fake signature of my mother. And the owner of the land is my mother but all the land related documents are with the party who gave hand loan. My question here is does my father has right to sell the property without my mother's involvement ? What is the validity of 20 rs Bond paper? Is 20 rs Bond valid without my mother's signature? Can I go legally without land related documents? If I go legally what is the scope of getting my mother's property? Does the party has legal right to aquire the property by showing that 20 rs Bond paper? Kindly suggest the next action