To set the stage, note that the Petterson court interprets Pattberg's offer as a unilateral offer, one that invites acceptance only by performance (paying of the mortgage early). See Offer and Acceptance Tutorial 3.
The court assumes that Petterson did not succeed in paying of the mortgage before Pattberg informed him that he had sold the mortgage to another. Recall the relevant rule from Offer and Acceptance Tutorial 4:
An offeree can no longer accept an offer if (1) the offeror acts in a way inconsistent with the intention to enter the bargain proposed in that offer, and (2) the offeree learns of the offeror's action from a reliable source. See Restatement (Second) Contracts ยง 43.
By selling the mortgage to another person, Pattberg acted in a way inconsistent with the intention of allowing Petterson to pay to Pattberg less than the amount due under the mortgage as payment in full.