Revocation?
It would seem to follow that Pattberg revoked the offer before Petterson could accept it.
Here is the reasoning: (1) the offer was a unilateral offer that could be accepted only by paying of the mortgage; (2) Petterson did not accept the offer before he learned from Pattberg himself that Pattberg had sold the mortgage to someone else; hence, (3) Pattberg succeeded in revoking the offer thereby terminating Petterson's power to accept.
This is in fact the reasoning of the majority in Petterson.