Is it permissible for someone in dire need of money to benefit from usurious financial gains, mixed with the principal, while ignorant of their exact amount, and then later donate their value or double it in charity?

Usurious gains must be disposed of by spending them on public welfare and acts of charity, such as giving to the poor and needy, and building mosques and hospitals. If you cannot accurately determine the amount of usurious gains, then endeavor to estimate an amount that you believe will cover them or exceed them. It is not permissible to benefit from these gains for yourself, unless you are poor.