In Islam, there are rules regarding a disbeliever who chooses Islam as his religion. An example of this is that if one shirks the rights of Allah, not performing obligatory actions or performing forbidden ones, he will be forgiven.
But, the rights of people which have been transgressed will not be forgiven; rather they must be observed or one must obtain the acceptance of those who had rights. So, if a person who has killed or raped someone else while he was a disbeliever and then becomes a Muslim he will only be saved from divine punishment, but he must bear the financial or physical punishments of this world, unless the inheritors forgive him. The reason behind this is that these rights do not have anything to do with Islam so that they will be forgiven by him becoming a Muslim; rather they have to do with people and are respected by every religion and by all rational people.
The Quran says the following about sins committed by disbelievers before they obtain faith: “O’ prophet, tell the disbelievers: ‘if you stop committing your bad deeds we will overlook your bad deeds of the past and forgive your sins.’”
The Prophet of Islam (s) said: “Islam covers up evil actions committed before one accepts Islam; repentance is similar to this in that it covers up evil actions, sins and disbelief.” This prophetic tradition is the base to the Islamic jurisprudential principle called the “Law of Jab”.
The meaning of the mentioned verse and tradition is that a person who intends to become Muslim should not worry about the bad actions that he performed in the past. Allah will forgive the person who shirked the divine rights in his period of disbelief. Therefore, it is not obligatory on him to perform the make-ups of various acts of worship or pay all of the zakāt that he did not pay in the past at one time because Allah is the oft-forgiving, the all-merciful and will forgive his own rights that he placed on his servant.
Likewise a person who has become a Muslim and performed an action which has a religious punishment while he was a disbeliever (for example, he drank wine) will not be punished. In the contrary, he will be treated with high levels of compassion and will not have the fear of being punished for his previous actions. Of course, this forgiving is in relation to Allah and Islam, but he must observe rights that people have over him or be punished for the murder or rape that he has committed against another person.
For example: if someone took a loan or stole he must return the goods back to their owner or achieve there satisfaction. Likewise, if he committed a bigger crime and injured or killed someone he is responsible for his actions and must bear their consequences. Allah forgives the rights that he has over people but not the rights that others have. So, the rights that were taken away from the people are preserved and must be made up.
It must be noted that this ruling is not exclusive to Islam; rather it is seen in all religions and bodies of law made by rational people.
Some scholars also add the following point that if a person becomes Muslim and people have so many rights over him that this new Muslim cannot observe all of them, the Muslim authority, if seen fit, can pay for them from the Muslim treasury, thus solving the problems of this new Muslim. For example, if he had to pay blood money from a murder that occurred when he was a disbeliever, the Muslim authority could pay it from the government’s budget.
In any case, this person has the right of punishment in any government, even an Islamic government because he caused oppression. In an Islamic government, one can sue the person in a righteous court and ask for punishment.
 Quran, 8:38
 Tarbihī, Majma‘ al-Bahrayn, under the word jabb; al-Sīrah al-Halabīyyah, v.3, p.105; This tradition has been narrated in different ways as well, for example: “Islam gets rid of what came before it,” Bihār al-Anwār, v.40, p.230.
 Refer to: Muhaqiq Hamadānī, Misbāh al-Faqīh, the chapter on zakāt, p.17; Muhammad Hassan Najafī, Jawāhir al-Kalām, v.17, p.15
 Refer to: Nāsir Mukāram Shīrāzī, al-Qawā’id al-Fiqīyyah, v.2, p.169-183 (“Law of Jab”)
 Abū al-Fath Hussaynī Jurjānī, Tafsīr Shāhī, v.2, p.96