Every person who shall act as a commissioner, being incapacitated or not duly qualified to act, or before he has made or subscribed such declaration as aforesaid, or after having become disqualified, shall for every such offence be liable to a penalty of fifty pounds; and such penalty may be recovered by any person, with full costs of suit, in any of the superior courts; and in every such action the person sued shall prove that at the time of so acting he was qualified, and had made and subscribed the declaration aforesaid, or he shall pay the said penalty and costs without any other evidence being required from the plaintiff than that such person had acted as a commissioner in the execution of this or the special Act; nevertheless all acts as a commissioner of any person incapacitated, or not duly qualified, or not having made or subscribed the declaration aforesaid, done previously to the recovery of the penalty, shall be as valid as if such person had been duly qualified.