Whether forgiveness by mistake (mechilah b'taut) is valid: Bava Metzia 66b-67a
Cases
Where one engaged in a deal which turned out to be an asmachta, and therefore invalid: Bava Metzia 66b
Where one attempted to purchase an item that does not yet exist: Bava Metzia 66b Overcharging for a sale: Bava Metzia 67a
