What is the ruling on brokering between two usurious banks for a commission, what is the ruling on the money earned from this brokering for those who work in it and for those to whom it is gifted, and does the ruling differ if the broker is Muslim or disbeliever?

The problem with the third bank taking a commission on the guarantees provided is that a guarantee is a gratuitous contract, not an exchange, so it is not permissible to take compensation for it. It is also not permissible for the company owner to act as an intermediary in this forbidden matter, whether for a fee or without a fee, because it constitutes cooperation in sin. Unlawful money is not owned by its possessor; rather, it must be disposed of by spending it on Muslim welfare or giving it to the poor and needy. There is no blame on the recipient for taking it.