22 C.F.R. § 34.7
Whenever feasible, and except as required otherwise by law, debts owed to the United States, together with interest, penalties, and administrative costs as required by this regulation, should be collected in one lump sum. This is true whether the debt is being collected under administrative offset, including salary offset, or by another method, including voluntary payment. However, if the debtor is financially unable to pay the indebtedness in one lump sum, payment may be accepted in regular installments. If STATE agrees to accept payment in installments, it may require a legally enforceable written agreement from the debtor that specifies all of the terms of the arrangement and which contains a provision accelerating the debt in the event the debtor defaults. The size and frequency of the payments should bear a reasonable relation to the size of the debt and ability of the debtor to pay. If possible, the installment payments should be sufficient in size and frequency to liquidate the Government's claim within 3 years.
22 C.F.R. §34.7