“If the customer initiates and asks, you are required to respond with proper advice,” replied Rabbi Dayan. “Also, if the customer is your relative, you can advise her even if she already decided to buy the item, since there is a priority to look out for the welfare of your relatives. In addition, if the storeowner is demanding an unfair price or cheating the customer on quality, you should inform the customer; this is not considered causing the storeowner damage. In all cases, though, you must be careful not to speak lashon hara about the store. You can say, ‘The item is cheaper at the other store,’ but should not say, ‘Prices here are always in the sky!’ ” (See Chofetz Chaim, Rechilus 9:10)
“Does it make a difference whether the store is Jewish owned?” asked Mrs. Cooper.
“If the store is not Jewish owned, it is permissible to alert a Jewish customer of possible saving,” replied Rabbi Dayan. “Although damaging a non-Jew is also prohibited, discouraging potential customers in this way is not considered causing damage in that case. Therefore, the mitzvah of hashavas aveidah to the fellow Jew remains.” (See Pischei Choshen, Geneivah 9:1; Nezikin 9:2)