Is it permissible for a person to pay off a debt for something haram that he purchased?

If the transaction is with a Muslim, it is impermissible to pay for something unlawful, because when Allah forbids a thing, He forbids its price. Furthermore, whatever the Lawgiver has forbidden its sale has no price. If one is coerced into paying, then he should pay and seek Allah's forgiveness and repent to Him from purchasing unlawful things. However, if the transaction is with a non-Muslim who does not consider what he sold to be unlawful, then payment is obligatory along with repentance, and the Muslim must cease dealing in unlawful things.