Is it permissible to pay a forbidden bribe to obtain a job that involves usurious money, knowing that the recipient of the bribe is sinful for taking it, whether the money is lawful or unlawful, and does this fall under the expenditures of usurious funds?

If you are the most qualified for the job and those in charge of hiring refuse to grant it to you unless money is paid, then there is no sin upon you for paying; because you are thereby attaining your right, and the sin falls upon the one who takes it.

However, what you pay must not be from usurious gains (riba), because it is not permissible to benefit from them personally. Rather, they must be spent on general Muslim welfare.