Sandra Offered to Purchase Jean's Car for $3,500, and Jean

Sandra offered to purchase Jean's car for $3,500, and Jean accepted this offer. This contract was reduced to writing and signed by the parties, when Sandra discovered that Jean was searching for an evening gown to wear to a social affair some months in the future. Sandra offered then to make a dress for Jean, if Jean would accept $3,200 for the car. Jean agreed to the modification and in return for Sandra's cheque in the amount of $3,200 gave her the keys to her car. Sandra finished the dress in time for the social event; however Jean was not pleased with the final result, and declaring it worthless to her, returned the dress to Sandra and brought an action on the face of the written contract for the $300 balance outstanding. Discuss the legal issues raised in this fact situation, its likely resolution, and treat explicitly how Sandra may counter in procedural terms with a defence against the document for $3,500, which is clear on its face and bears her signature.
D.L.R. 985.