If the property in their possession, or in the possession of others, is known to have been forcibly taken from a person protected by Islamic law, it is not permissible to purchase it in order to acquire it as one’s own. However, it is permissible to purchase it as a means of recovering it for its lawful disposal: returning it to its owners if possible, or otherwise using it for the benefit of the Muslims.

If it is known that their wealth includes something unlawful, but the particular property cannot be identified, this does not make dealings with them unlawful. The same applies when it is known that the marketplace contains property that has been forcibly taken or stolen, but that property cannot be identified. When the unlawful becomes mixed with the lawful, there are two cases:

The first concerns what is prohibited in itself, such as carrion or marriage to a sister through breastfeeding. If the prohibited item or person is indistinguishable among a number too large to enumerate, this does not make the others unlawful. For example, a man may know that he has a sister through breastfeeding in a particular town without knowing who she is, or that someone there sells carrion without knowing which meat it is. This does not make the women or meat of that town unlawful to him. However, if he cannot distinguish his sister from one unrelated woman, or lawfully slaughtered meat from carrion, both alternatives are prohibited.

The second concerns property that is unlawful because it was forcibly taken, or was received through unlawful transactions such as usury and gambling. If such property becomes indistinguishably mixed with other property, the whole does not become unlawful. Rather, the respective amounts must be distinguished, and each portion allocated to whoever is entitled to it. An example is a thief who takes people’s property and mixes it together, or takes their wheat or flour and mixes it together. It is then divided among them in proportion to their respective entitlements.