In that case, the buyer would be entitled to rescind the sale or to trade the item of lesser value for the item originally bargained for, or to recoup the difference between the value received and the value for which he paid.
Did the gallery provide what it guaranteed?
No. The seller actually provided something worth much more than the item it originally guaranteed, and the mistake inured to the benefit of the buyer rather than to his detriment. There is no cause of action for accidentally benefiting the other party.
What about the element of duress?
There is no issue of duress. The requirements of duress are not met where the putative "duress" relates to something that is inconsequential like the type of threat made by the seller here. Even if the threat was credible, the requirements of duress are not met where the supposed harm threatened is capable of redress through ordinary civil procedures. In this case, if the seller made good on his threat to badmouth the buyer, the buyer could recover against him by showing that he lost out...
To claim duress, the buyer would have to have been threatened with bodily injury or other credible threat to his (or another) person.
Is the gallery director potentially liable to the defense of duress if you agree to rescind the contract and return the painting (and hopefully receive a return of the monies you paid).
No, for the reasons expressed above. The buyer is not under any obligation to rescind the transaction that resulted from the seller's unilateral mistake, particularly since the buyer was completely oblivious to the mistake at the time. Unless the buyer was threatened with bodily injury or other similarly dangerous action that would cause a reasonable person to fear for his safety (or another's), he has no claim for duress.
If the buyer chooses to agree to rescind the transaction, he is entitled to that for which he originally believed he was purchasing or its value.
