Is it permissible to take from money about the lawfulness of which there is a difference of opinion (such as the money of a barber who shaves beards, and the money of one who uses musical instruments) for a building for which it was stipulated that its funds must be lawful, given the scholarly disagreement regarding the lawfulness of such money?

If a person stipulates that his partner's money must be lawful according to Islamic law, not merely what the partner considers unlawful, then it is permissible to accept money from someone who acquired it through an action over which there is a difference of opinion regarding its permissibility. This is because such money is considered lawful based on thewer's belief, derived from a permissible ijtihad (independent reasoning) or a legitimate taqlid (following a qualified scholar). The proof for this is that dealings are permissible concerning matters whose unlawfulness is not known. Furthermore, contracts over which there is a difference of opinion are no worse than the sale of intoxicants by disbelievers, the proceeds of which are permissible to deal with. If a Muslim believes his earnings to be permissible, then the money is excused for him, even if it transfers to another person through inheritance or a gift. This is supported by the saying of Ibn Mas'ud (may Allah be pleased with him): "Its permissibility is for you, and its sin is upon him."