S.D. Codified Laws § 20-4-8
Current with legislation signed by the governor on or before 3/6/2024
If no application be made pursuant to § 20-4-7, the creditor, within a reasonable time after such performance, may apply it toward the extinction of any obligation, performance of which was due to him from the debtor at the time of such performance; except that if similar obligations were due to him, both individually and as a trustee, he must, unless otherwise directed by the debtor, apply the performance to the extinction of all such obligations in equal proportion; and an application once made by the creditor cannot be rescinded without the consent of the debtor.
SDCL 20-4-8