Regarding Fake School Transfer Certificate

Actually my father obtained school transfer certificate in 2008 from school clerk of class 6th pass on which there was seal of school principal and Block education officer which the clerk forged. When the principal came to know about this, my father was called in school and after discussion the matter was mutually settled in year 2008 itself. But now in 2020, the school principal is threatening to file a FIR since he didn't gave in written about the settlement at that time. Is it legal to file case after mutual settlement ? How can I save my father ? Please help I am mentally distressed.We are very poor family.