Where a contract provides an exclusive method by which its consideration is to be ascertained, which method appears possible on its face but in fact is or becomes impossible of execution, such provision only is void.

Source: CivC 1877, § 914; CL 1887, § 3538; RCivC 1903, § 1232; RC 1919, § 847; SDC 1939, § 10.0505.