Master good faith and fair dealing in contract performance.
Market Street Associates Limited Partnership v. Frey, 941 F.2d 588 (7th Cir. 1991)
A lease contained a clause allowing the lessee to buy the property at a favorable price if the lessor refused to make financing available. Market Street triggered the clause by requesting financing they knew would be refused, without telling the lessor about the favorable buy-out provision. Judge Posner examined whether this violated good faith.
The duty of good faith prohibits taking advantage of the other party's oversight of contract terms. While parties needn't be altruistic, they must not exploit gaps in the other's knowledge of contract provisions, particularly in ongoing relationships where trust is expected.
This Posner opinion is the leading modern case on good faith and fair dealing. It established that good faith means honesty in fact and reasonable commercial standards, requiring parties not to exploit oversights while still allowing vigorous pursuit of contractual rights.