Is interceding between two parties in the financing of one party by the other, in exchange for a commission, permissible in Islamic law, given that the intercessor is aware of the existence of a suspicion of usury (riba) in the transaction?

The permissible murabaha is when the financier buys a commodity, takes possession of it, and then sells it to your friend in installments with an agreed-upon profit. However, if the financier gives the amount directly to your friend, with the condition of receiving it back with an increase, then this is an usurious loan, which is unanimously forbidden. Naming it "financing" or "murabaha" is a deception and a forbidden trick. It appears from the question that the transaction is not a legitimate murabaha due to the absence of commodities. If the transaction is an usurious loan, then your mediation in it is forbidden, and you must repent to Allah and get rid of the money resulting from this transaction.