FIR sec 3 & 4 dp act, sec 504 & 506 uttar pradesh

Sir, A fake fir has been registered against me and my family. Allegations - marriage was fixed and girls family gave us 1 lakh rupees in july 2017. But now due to increase in dowry demand we are cancelling marriage. Truth is we just saw the girls family in july 2017. There was no conversation for the whole year. Fir registered in aug 2018. Now investigation officer is making another story that I was in live in relationship with that girl. May be he has taken bribe and saying this to us. Truth is I saw girl only once in july 2017. My question is ,in original complaint there was no allegation related to live in relationship, can the fir be modified again and fresh charge of live in relationship can be levied against me? Sir kindly help please.