How can one abstain from financial dealings mixed with the unlawful, given their prevalence, and does the required piety reach this extent, making life difficult?

Piety is commendable as long as it does not lead to hardship. Dealing with people whose wealth is mixed is permissible. If someone performs an act that they believe to be lawful, it is permissible to deal with them using the money earned from that act, even if the person dealing with them considers the act unlawful. This is based on the statement of Sheikh al-Islam Ibn Taymiyyah: "If a Muslim engages in a transaction that they believe to be permissible, and receives money, it is permissible for other Muslims to deal with them concerning such money, even if they do not believe that transaction to be permissible."

It is important to differentiate between unlawful money obtained through theft, usurpation, and similar means – with which it is not permissible to deal for one who knows its nature – and money obtained through prohibited transactions like usury. Some scholars have held that it is permissible to deal with the owner of such money and benefit from it, as stated by Sheikh Ibn 'Uthaymeen: "What is prohibited due to its earning is prohibited only for the earner. As for what is prohibited due to its essence, it is prohibited for both the earner and others." He supported this by stating that the Prophet, peace and blessings be upon him, used to deal with Jews and eat from their food, despite their involvement in usury. Accordingly, it is permissible to take from a father's money earned from usury.