S.D. Codified Laws § 7-19-8

In all cases where a board of county commissioners of any county has, prior to July 1, 1935, effected a compromise settlement of any indebtedness secured by a state permanent school fund mortgage, no action or proceeding shall be maintained to invalidate or set aside such compromise settlement or the satisfaction of such mortgage on the ground of lack of authority of such board to effect such compromise settlement, any existing statute to the contrary notwithstanding.