rules for figuring out the unstated interest IRS: Publication 537 -- Installment Sales installment sale IRS Installment Sales Contract Guidelines recognized by the IRS Publication 537 (2009), Installment Sales IRS.gov: Publication 537 Publication 537, Installment Sales IRS: Publication 537 - Installment Sales installment sale IRS.gov: Publication 537 (2010), Installment Sales rules for figuring out the unstated interest