N.D. Cent. Code § 9-12-05: Partial performance - Extinction pro tanto

A partial performance of an indivisible obligation extinguishes a corresponding proportion thereof if the benefit of such performance is retained voluntarily by the creditor, but not otherwise. If such partial performance is of such a nature that the creditor cannot avoid retaining it without injuring the creditor's own property, the creditor's retention thereof is not presumed to be voluntary.