Unless otherwise exempted, the wages and salaries of all employees of this Commonwealth, other than State officers, shall be subject to garnishment or execution upon any judgment rendered against them. Whenever the salary or wages of such employees as above mentioned shall be garnished under this section, the process shall be such as is usual in other cases of garnishment and shall be served on the judgment debtor and on the officer or supervisor who is head of the department, agency, or institution where the employee is employed, or other officer through whom the judgment debtor’s salary or wages is paid, provided that process shall not be served upon the State Treasurer or the State Comptroller except as to employees of their respective departments, and upon such service the officer or supervisor shall, on or before the return day of process, transmit to the clerk of the court issuing the process a certificate showing the amount due from the Commonwealth to such judgment debtor, up to the return day of the process, which amount the officer or supervisor shall hold subject to order of the court issuing the process. Such certificate shall be evidence of all facts therein stated, unless the court direct that the deposition of the officer or supervisor, or such other officer through whom the judgment debtor’s salary or wages be paid, be taken, in which event the deposition of the officer or supervisor shall be taken in his office and returned to the clerk of the court in which the garnishment is, just as other depositions are returned, and in no such case shall the officer or supervisor be required to leave his office to testify. In all proceedings under this section, if the judgment be for the plaintiff, the amount found to be due the judgment debtor by the Commonwealth shall be paid as directed by the court.