Q: I gave a jeweler a diamond to sell for at least $10,000. Two months later, he sold it for $9,000, claiming that it is worth only that much and he cannot get the price I requested. What is the halachah?
A: A broker is an agent of the owner/seller. Therefore, he is not allowed to deviate from the instructions of the seller, even though his intention is for the benefit of the seller, and even if he knows the market conditions better (C.M. 185:1).
Therefore, if the owner set a minimum price, the broker may not sell for less, even if he was unable to sell at the price the owner stated. If the sale cannot be voided, the broker is required to compensate the seller for the difference (Sma and Aruch Hashulchan 185:1; Nesivos 185:2).
Conversely, if the broker sold at a price higher than was set, even if the owner had agreed to sell below value to raise cash, the gain is the owner’s.