Since 1964, a minimum interest rate has been imposed on installment sales. More precisely, a part of each principal payment under an installment sale is treated as interest rather than sales price (and the total sales price is correspondingly reduced) if interest of less than the prescribed rate is specified.
Harl, Neil E.
"Interest Rates on Installment Sales,"
Agricultural Law Digest: Vol. 2
, Article 1.
Available at: http://lib.dr.iastate.edu/aglawdigest/vol2/iss16/1