Someone bought something from me but he did not pay . I learned later that all the money he has is Haram. How I get my Halal money from his Haram money?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is no harm in taking your money back from this person and you do not commit a sin if you do that Insha Allah.
The Prophet Muhammad (Blessings and peace of Allah be upon him) used to deal with Jews and other non-Muslims. He used to sell them things and to buy from them. It is well known that these people earned most of their livings through unlawful means such as Riba, gambling and selling alcohol.
But you should advise this man and make clear to him how dangerous it is to earn one's living through unlawful means. Tell him that he will be asked on The Day of Judgment about his money and wealth how he earned them and how he spent them as narrated in a sound Hadith.

Allah knows best.