The World Federation One Stop Fiqh

Ruling 1832

If lawful property becomes mixed with unlawful property and one does not know the quantity of the unlawful property but does know who its owner is, in the event that the person and the owner cannot come to a mutual agreement [as to the quantity of the unlawful property], the person must give the owner a quantity that he is certain is his. In fact, if the person himself was at fault in the two properties – i.e. the lawful and the unlawful – becoming mixed, then as an obligatory precaution he must give him more than what he deems probable is his property