Even if in writing, such a promise to reconvey would not be specifically enforced for the reason that it was unilateral, as plaintiffs do not show that they agreed to take back the property. ( Levin v. Dietz, 194 N.Y. 376; Riker v. Comfort, 140 App. Div. 117.) These appellants cannot avail themselves of the alleged usury in giving a mortgage before they acquired the property.