SDCL § 20-2-6: Performance of conditions waived by notice of nonperformance by other party.

If a party to an obligation gives notice to another, before the latter is in default, that he will not perform the same upon his part, and does not retract such notice before the time at which performance upon his part is due, such other party is entitled to enforce the obligation without previously performing, or offering to perform, any conditions on his part, in favor of the former party.