Praise be to Allah.
Firstly:Who has guided him to Islam. We ask Allaah to make him steadfast and to guide him to that which is good for him in this world and in the Hereafter.
By His grace and mercy, Allaah has caused Islam to destroy the sins that came before it. When a disbeliever becomes Muslim, Allaah forgives all that he did when he was a non-Muslim, and he becomes cleansed of sin.
Muslim (121) narrated that ‘Amr ibn al-‘Aas (may Allaah be pleased with him) said: When Allaah put Islam in my heart, I came to the Prophet (peace and blessings of Allaah be upon him) and said: “Give me your right hand so that I may swear allegiance to you.” He held out his hand and I withdrew my hand. He said, “What is the matter, O ‘Amr?” I said, “I want to stipulate a condition.” He said, “What do you want to stipulate?” I said, “That I will be forgiven.” He said, “Do you not know that Islam destroys that which came before it?”
“Islam destroys that which came before it” means that it erases it and wipes it out. Stated by al-Nawawi in Sharh Muslim.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked a similar question, about someone who earned money by dealing in drugs before he became Muslim. He replied:
We say to this brother whom Allaah has blessed with Islam after he had earned haraam (unlawful) wealth: be of good cheer, for this wealth is permissible for him and there is no sin on him in it, whether he keeps it or gives it in charity, or uses it to get married, because Allaah says in His Holy Book (interpretation of the meaning):
“Say to those who have disbelieved, if they cease (from disbelief), their past will be forgiven. But if they return (thereto), then the examples of those (punished) before them have already preceded (as a warning)”
i.e., all that has passed, in general terms, is forgiven. But any money that was taken by force from its owner must be returned to him. But money that was earned by agreements between people, even if it is haraam, like that which is earned through riba (interest)or by selling drugs, etc. it is permissible for him when he becomes Muslim because Allaah says, “Say to those who have disbelieved, if they cease (from disbelief), their past will be forgiven”. And the Prophet (peace and blessings of Allaah be upon him) said to ‘Amr ibn al-‘Aas when he became Muslim: “Do you not know that Islam destroys that which came before it?”
Many of the non-Muslims became Muslim after they had killed Muslims, but they were not punished for what they had done. So tell this brother that his wealth is permissible and there is no sin in it; he may give it in charity and use it to get married. As for what he has been told about it not being permissible for him to give it in charity, there is no basis for that. End quote.
Liqaa’aat al-Baab il-Maftooh, /373-374.