Option contracts are not the only way in which offers may become irrevocable for a certain period of time. The offere's reliance on the offer may make the offer irrevocable. Petterson v. Pattberg, 161 N.E. 428 (N.Y. 1928), illustrates the point.
Petterson owned a building on which Pattberg held the mortgage. On April 4, Pattberg offered to accept significantly less than the total amount remaining amount due as payment in full if Petterson paid off the mortgage by May 31. After raising the necessary funds, Petterson took the money in person to Pattberg's house; however, before Petterson could hand the money to Pattberg, Pattberg told him he had sold the mortgage to another.
The question in the case is whether Pattberg succeeded in revoking the offer to Petterson prior to acceptance. The court holds that he did.
But: Petterson is a 1928 case, and the law has changed.
Under the modern law, a court might hold that Petterson's reliance on the offer made it irrevocable. The following question and answer sequence illustrates the reasoning that underlies the modern treatment.