What is the ruling on the surplus money I received as a result of the intermediary company defrauding the granting company, given my knowledge of the fraud and my inability to return the money or inform the granting company?

It is permissible for someone who has contracted to perform a work to hire another to do it for a lower wage, unless the agreement specified that he should perform the work himself for a specific purpose.

If the one who gave you the work defrauded [the client] and took more than he deserved, and gave you a portion of it, then only what you agreed upon, or the customary wage if no agreement was made, is lawful for you. You are obligated to return the excess money to the granting company or work for them without charge for a period equivalent to their money. If this is not possible, then donate it as charity with the intention that the reward goes to its rightful owners.