What is the ruling on taking out a loan with interest from a bank, then having a mediator waive the interest in exchange for a percentage of the loan?

It is not permissible to take a usurious loan, even if someone is found to cancel the interest after the contract, because the contract is initially invalid and involves an acknowledgment of usury. The intermediary may also be unable to cancel it, in which case you would be bound by the usurious interest, thus becoming one who deputizes usury. The Prophet, peace and blessings be upon him, said: "Allah has cursed the one who consumes usury, the one who deputizes it, its witness, and its scribe. They are all equal in sin."