SDCL § 53-11-7: Extinction by intentional destruction, cancellation or alteration of contract--Alteration or destruction of duplicate, not to prejudice.

The intentional destruction, cancellation, or material alteration of a written contract by a party entitled to any benefit under it, or with his consent, extinguishes all the executory obligations of the contract in his favor against parties who do not consent to the act. Where a contract is executed in duplicate, an alteration or destruction of one copy while the other exists is not within the provisions of this section.