He has a debt for
    something haraam – should he repay it?
Question

                Is it permissable for someone to
      repay a debt on something that was haram?jazakallah khairyn.
Praise be to Allah.He should look at this forbidden thing and if the
    transaction was with a Muslim, then it is not permissible for him to pay the debt, because
    when Allaah forbids a thing He also forbids its price, and because whatever it is
    forbidden to sell in Islam has no price. The Prophet 
 (peace and blessings of Allaah be
    upon him) forbade the price of a dog and said: “If he comes asking for its price,
    then fill his palm with dust.” This is reported in Al-Saheeh from the hadeeth
    of Ibn Abbaas.
It was reported from Abu Mas’ood al-Ansaari (may Allaah be pleased
    with him) that the Messenger of Allaah 
 (peace and blessings of Allaah be upon him)
    forbade the price of a dog, the earnings of a prostitute and the fee (hulwaan) of a
    fortune-teller (narrated by al-Bukhaari, 2083). The word hulwaan
    comes from the phrase “halawtuhu hulwaanan”, meaning “I gave it to
    him”; the root of this word is al-halaawah (meaning “sweet”) and it
    is likened to something sweet because he takes the money so easily, without having to work
    hard or make any effort… Hulwaan also means “bribe.”
    But if a person is forced to repay the debt, then in this case he
    should pay it whilst seeking the forgiveness of Allaah and repenting to Him for buying
    haraam things, whether from a Muslim or anyone else.
If the transaction was with a non-Muslim in whose religion the thing he
    sold is not viewed as forbidden, then the debt has to be paid whilst also repenting. The
    Muslim has to avoid dealing in things that Allaah has forbidden.