Recently my died father gave empty false promiseery note and came to know from lender after death

My father died recently. He is holding some properties and having some debth. Some small debth, i came to know before my father died and they hold promiseery note and i promised to pay them. The problem is my father's friend is holding empty promiseery note and started infoming after death of my father and didn't bring into notice when he was there to question my father. His information is looking false to me to believe either. He is saying that my father took 13 lakhs, past 10 years back and saying that my father told not tell everyone and he is telling not done any transaction regarding it. Secondly , my father meet with an accident and lost his right signature hand two years back and can't do right hand signature from past two years. I tried to negotiate with him for time and to collect information regarding financial transactions with my father's friend.If it true to pay him in interval atleast. The lender is forcing to give my promiseery note and he is trying to sending legal notice which i did not take into consideration for first time . He is trying to take court opportunity and misuse it , to get claimed from transferring my father's property which will be transferred to me after bank loan clearance. He is trying to take opportunity of blank empty note to use three years before my father died , just one year before my father meet accident and only signature of my father remaining but rest is all filled by his handwriting. I am trying to negotiate with my elders in the family before sending second time legal notice from him . I can face him in the court but not interested without law awareness and to spoil my family name. Please give me proactive law advice, how to solve this problem. Negotiation , required fully amount to pay with my promiseery note in writing.