Is it permissible to apply for a financial grant from a private company whose source of profit is unknown, taking into account that the majority of similar companies are lenient in usurious dealings?

If unlawful wealth is mixed with lawful wealth, and the entire wealth is not unlawful, then dealing with it is not prohibited. Muslims, since the time of the Messenger of Allah, peace and blessings be upon him, have continued to deal with polytheists in buying, selling, and other transactions, even though the polytheists do not refrain from dealing in usury and gambling. There is no harm in requesting a grant from a company if it is not known that all its wealth is unlawful.