How does one calculate the loss from lawful or unlawful money when selling something that was purchased with mixed funds, and its owner now wishes to repent and purify his wealth?

If a Muslim's wealth is mixed with unlawful (haram) money, he must extract the amount of unlawful money from his wealth and disburse it in its Sharia-prescribed channels. The loss he incurred from his lawful (halal) money remains. Sheikh al-Islam Ibn Taymiyyah was asked about a man whose lawful wealth is mixed with unlawful wealth, and he replied: "He should extract the amount of unlawful wealth by measure and give it to its owner, and the amount of lawful wealth is his."