When a non-Muslim embraces Islam, it is very likely that some portion of his wealth has come from sources that Islam considers illegitimate. For example, the convert could have money that resulted from interest-bearing transactions and investment, selling or serving alcohol and so on. What should the new Muslim then do with such wealth that is already in his possession?100
The general rule is that any wealth that one has in one's possession at the time of conversion remains the property of the convert regardless of how that wealth was gained, as long as it was gained in a legal fashion according to the laws the convert was living by. The individual is not held responsible for his lack of applying Islamic principles prior to his conversion. Thus, for example, Allah says, “Those who after receiving direction from their Lord, desist, shall be pardoned for the past” (2:275). This verse demonstrates that Allah overlooks the actions that one performs before the rulings reach him and he is obligated to follow such regulations.
Numerous people embraced Islam during the lifetime of the Prophet (peace and blessings of Allah be upon him) yet there is no record of him asking any of them about the wealth in their possession and how they obtained such wealth. Indeed, even marriages that took place before the conversion were not questioned or examined to see if their contracts met the Islamic standards. In fact, there are various reports that demonstrate that the Prophet explicitly approved of the converts keeping the wealth in their possession. Actually, the person earned such wealth believing that there was nothing wrong with what they were doing. Hence, they are allowed to keep such wealth. Their case is different from a Muslim who knowingly deals in alcohol, for example. Such a Muslim, even after repenting from such an act, is not to keep that ill-earned wealth.
However, the situation is different if the convert has, at the time of his conversion, not yet received money that is from a source that Islam considers illegitimate. For example, the individual could have sold and delivered someone alcohol on July 1 but the agreement between them is that he is not to be paid un l December 1. In the mean me, say in September, the one who sold the alcohol converts to Islam. It is possible to look at this and say that since the contract was concluded before his conversion, he is still entitled to this money, as this is wealth he earned before becoming Muslim.
However, the majority of the scholars state that he no longer has the right to that money. They quote, “Those who after receiving direction from their Lord, desist, shall be pardoned for the past” (2:275) once again. Now, the admonition has come to him and he can only keep what he received earlier and must forego anything additional. Allah also says, “If you repent, you shall have your principle” (2:279). Thus, for example, the Prophet (peace and blessings of Allah be upon him) abolished all of the interest-bearing agreements during a speech in Makkah after many people had just embraced Islam. Hence, although those contracts were concluded before they had embraced Islam, the Prophet (peace and blessings of Allah be upon him) clearly voided the forbidden aspect of the contract.
In sum, once an individual embraces Islam he should from that moment on forego and not accept any wealth that is earned through forbidden means, regardless of whether the contract for that wealth took place before his conversion. Actually, now the individual should believe that such money is forbidden and therefore he himself should no longer wish to receive it or benefit from it. Given the nature of contracts nowadays, he may not be able to cancel the contract. If he is forced to receive such money, he should give it away and free himself from it. (Many mosques have specific accounts for monies received through illegitimate means but which one is forced to receive, such as interest on deposits, and will use that money is very specific ways as recommended by the scholars.)
100 For more details on this issue, see Abbaas al-Baaz, Ahkaam al-Maal al-Haraam (Amman, Jordan: Daar al-Nafaa`is, 1999), pp. 121-134. Al-Baaz’s work is the main reference for this section.