Interest at the rate of one percent per month, or fraction thereof, shall accrue on debts owed to the state, starting on the date the debts become past due. This section does not apply to: (1) Any instance where such interest rate would conflict with the provisions of a contract or with the provisions of any other law; or (2) debts to be paid by other governmental units. The office of financial management may adopt rules specifying circumstances under which state agencies may waive interest, such as when assessment or collection of interest would not be cost-effective. This section does not affect any authority of the state to charge or collect interest under any other law on a debt owed to the state by a governmental unit. This section applies only to debts which become due on or after July 28, 1991.