Should there be, at the time of the passage of this act, a contract or agreement, in writing, existing between any corporation, firm, or individual owning the water plant or system, and a municipality then being supplied with water by such corporation, firm, or individual, establishing or adjusting or fixing the manner in which such water plant or system may be appraised, purchased, or acquired, then, and in that case, appraisers shall be selected, and the appraisement made in accordance with the terms of such contract, and to the respective parties to such contract shall, in lieu of the preceding provisions of this act, carry out the terms of said contract in arriving at the value of said water plant or system.
53 P.S. § 2967