31 CFR §205.15

1. (a)General rule. State interest liability may accrue if Federal funds are received by a State prior to the day the State pays out the funds for Federal assistance program purposes. State interest liability accrues from the day Federal funds are credited to a State account to the day the State pays out the Federal funds for Federal assistance program purposes.
2. (b)Refunds.
   1. (1)A State incurs interest liability on refunds of Federal funds from the day the refund is credited to a State account to the day the refund is either paid out for Federal assistance program purposes or credited to the Federal government.
   2. (2)We and a State may agree, in a Treasury-State agreement, that a State does not incur an interest liability on refunds in refund transactions under $50,000.
3. (c)Exception to the general rule. A State does not incur an interest liability to the Federal government if a Federal statute requires the State to retain or use for Federal assistance program purposes the interest earned on Federal funds, notwithstanding any other provision in this section.
4. (d)Mandatory matching of Federal funds. In programs utilizing mandatory matching of Federal funds with State funds, a State must not arbitrarily assign its earliest costs to the Federal government. A State incurs interest liabilities if it draws Federal funds in advance and/or in excess of the required proportion of agreed upon levels of State contributions in programs utilizing mandatory matching of Federal funds with State funds.