Is one allowed to pay a service fee for an aveirah which someone else has already done, in order to get some other good or service now? The case I had in mind is an animal adoption where the agency charges a fee for neutering already performed.
Paying for an Aveira that was already done, is probably not a problem - especially in this case where you did not request the Aveira to be done.
In הלכות איסורי מזבח פרק ד:יב the Rambam states that if you pay a harlot after the fact, the payment does not have the status of Ethnan and can be used as a Korban. This is true even if you agreed on a price and only sent the animal later, as Yehuda did (or tried to do) with Tamar.