It is a general rule of law that oral agreements that materially modify a written agreement within the Statute of Frauds are not enforceable. Tex.Bus. & Comm.Code Ann. § 26.01; King v. Texacally Joint Venture, 690 S.W.2d 618, 619 (Tx. App.--Austin, 1985, writ ref'd N. r.e.); Dracopoulas V. Rachal, 411 S.W.2d 719 (Tex. 1967). However, not all modifications are prohibited. If the oral changes do not materially alter the underlying obligations, for example, they are not barred. Horner v. Bourland, 724 F.2d 1142, 1148 (5th Cir. 1984); Group Hospital Services, Inc. v. One and Two Brookriver Center, 704 S.W.2d 886, 890 (Tx. App.--Dallas, 1986, N. w.h.).
Courts have also adopted the doctrine of promissory estoppel in some cases to forbid reliance on the Statute of Frauds as a defense to the validity of oral agreements. In specific, where one party reasonably relies on the oral promise of another to reduce an oral agreement to writing, the failure to create such a writing will not prevent the relying party from taking the modification out of the Statute of Frauds. The Court specifically invoked both §§ 90 and 178, comment F of the Restatement, ...