In the case mentioned in the question the seller's broker violated their fiduciary obligation to the seller because they have acted in excess /beyond their authority
Explanation:
A broker who is representing the seller has taken a listing on a property at a price of $400,000. The broker showed the property to a potential buyer and the buyer made an offer of $375,000, but told the broker that he/she would pay the full $400,000 if necessary.
After analysis the below mentioned statement from the question we can view the following facts
Hence we can say that the seller's broker violated their fiduciary obligation to the seller because they have acted in excess /beyond their authority