N.Y. UCC Law § 3-106

Section 3--106. Sum Certain.\n (1) The sum payable is a sum certain even though it is to be paid\n (a) with a stated rate of interest or by stated installments; or\n (b) with stated different rates of interest before and after\n default or a specified date; or\n (c) with a stated discount or addition if paid before or after\n the date fixed for payment; or\n (d) with exchange or less exchange, whether at a fixed rate or at\n the current rate; or\n (e) with costs of collection or an attorney's fee or both upon\n default.\n (2) For the purposes of subsection one of this section "a stated rate\nof interest" shall also include a rate of interest that cannot be\ncalculated by looking only to the instrument but which is readily\nascertainable by a reference in the instrument to a published statute,\nregulation, rule of court, generally accepted commercial or financial\nindex, compendium of interest rates, or announced rate of a named\nfinancial institution.\n (3) Nothing in this section shall validate any term which is otherwise\nillegal.\n