Notwithstanding any law to the contrary, the commissioner may employ as witnesses to testify in eminent domain proceedings such persons who are qualified to give competent testimony as to the value of the property, or estate, right or interest therein, being condemned in an eminent domain proceeding, as he deems necessary. Each such person so employed shall receive for his services the sum agreed upon in a written contract signed by the commissioner and such person. The sum contracted to be paid, however, shall not be made contingent upon the outcome of the action or hearing in which the competent testimony is to be given.