N.Y. Gen. Oblig. Law § 5-1111

§ 5-1111. Execution by agent in real property transactions; written\nauthorization required. If executed by an agent, any agreement,\npromise, undertaking, assignment or offer required by section 5-1103,\n5-1105, 5-1107 or 5-1109 to be in writing, which affects or relates to\nreal property or an interest therein in any manner stated in\nsubdivisions one or two of section 5-703 of this chapter, shall be void\nunless such agent was thereunto authorized in writing.\n