What exactly the client is buying, who supervises the specialist day to day, and who is responsible for the outcome. Those answers determine whether the placement reads as a service or as labour for hire.
Negotiate the clause, not the explanation
Filing fees are published in the USCIS fee schedule and are the small number here. The costly scenario is a signed agreement whose terms hand direction of the specialist to the client, discovered when the petition is being prepared. Renegotiating at that point is awkward and sometimes impossible, whereas raising it during drafting is routine.
The questions to put are concrete: is the client purchasing a defined deliverable or a person's time, who assigns daily work, who the specialist reports to, and who bears responsibility if the work is defective. A Douro-Dummer firm should also ask internally whether it is willing to carry that responsibility, since retaining control is a commercial commitment as well as an evidentiary one and it has to be real rather than drafted.