I see. Thank you for clarifying this for me, Robert.
Under TX law, in order to rescind a contract based on the doctrine of "unilateral mistake", the party seeking to rescind the contract must typically prove: (1) the mistake must be "of so great a consequence that to enforce the contract as made would be unconscionable"; (2) it must "relate to a material feature of the contract"; (3) it "must have been made regardless of the exercise of ordinary care"; and (4) "the parties can be placed in status quo in the equity sense." Taylor v. Arlington Indep. Sch. Dist., 335 S.W.2d 371, 373 (Tex. 1960); see Zapatero v. Canales, 730 S.W.2d 111, 114 (Tex. App.--San Antonio 1987, writ ref'd n.r.e.).
Here is a link which sets out these elements:
Therefore, unless the seller can show that the contract as written is unconsionable, they would typically be bound by their employee's mistake. If you receive a really good deal, that would not be enough normally, but the seller must prove that to enforce the contract would be extremely unfair. It is not likely they can prove this, so they would normally be bound by the employee's mistake.
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