He found out after marriage that his wife is mentally ill. Should he ask
    for the mahr to be returned?
Question

                I married and later on I discovered my wife
      has psychological problem. Her other sister are ok. They only agree to have MEHR_Missal at
      that time (as I do not about her problem). I already give her MEHR. 
      Under such condition what MEHR suppose to be. I feel is a total fraud. 
      Is Mehr suppose to be justified again? 
      Although, I am on terrible road of marriage life. 
      It is probably possible that (if there is a separation) she can use that same MEHR money
      against me by hiring good lawyer and get most out of the USA law (against me). What I can
      do in this situation?
      Jazakallah Khair!
Praise be to Allah.Refer to a sharee’ah judge (qaadi) if there is any
    qaadi there. If not, then if the man has consummated the marriage with the woman, the mahr
    must be given to her because of having had intercourse with her. As for this sickness, if
    it prevented him from consummating the marriage or put him off from doing that properly,
    then whatever was paid to the one who tricked him into this marriage should give it back,
    if he was tricked. If that is not the case, then he does not deserve anything because he
    did not ask enough questions and find out information properly.