ANSWER:

You aren’t married and hence being in contact with one another is sinful 

If your father agrees, you can have the contract done without his parents knowing about it and you are now man and wife and can talk and communicate without being sinful 

QUESTION: We used to like each other, hence our parents decided to solidify the relationship, Recently, we got engaged However, my fiancé and I have always wanted a Nikkah. He asked his parents several times to have our Nikkah done, but they think that Nikkah without rukhsat is senseless, as no one knows what the future holds. In this case, are we sinful?