Is the money you obtained from the brokerage permissible, even though the other party claims they paid it for voluntary work and denies the brokerage?

If a specific commission for a particular task was agreed upon and the task was completed, then the commission is due to you. If you cannot find a way to collect it and you come across some of the owner's money, it is permissible for you to take what is rightfully yours from it. This is known as the issue of "al-Dhafr" (seizing one's right). This is not treachery; rather, it is attaining one's right, as stated by Al-Qurtubi. Therefore, if your right to the commission is established due to the completion of the work, then there is no blame upon you for what you took from the money, provided it is equal to your agreed-upon right; otherwise, you must return the excess.