Bava Batra 83 - Four Types of Errors in Sales
If the seller sold what was purported to be superior wheat, and it was found to be inferior, the buyer can retract. The seller might also want to void the sale if the price rises, but he does not have this option. It is not the case of a fraudulent sale, where both can retract. Rather, it is more akin to a case of price fraud, or overcharge , since the buyer did receive wheat, albeit of inferior quality.
Similarly, if he sold him inferior wheat, and it was found to be superior, the seller can retract.
The "red" wheat is reddened by the sun, and it produces more flour, while the "white" wheat yields superior bread. If one agreed to sell red wheat and it was found to be white, or vice versa, both can retract.