Hilchos Rechilus, chapter 9, section 15

Now we will explain something many people stumble in. I will bring one case, and from it a wise person can learn about similar cases.

It often happens that someone brings goods to a town to sell, and many buyers come to him. Many times, a buyer who does not have money with him asks the seller to keep the goods aside for him until he comes back with money. Meanwhile other buyers come and push the seller to sell them the goods the first buyer chose, and he agrees. Later, when the first buyer comes back with money, the seller tells him: "So-and-so came and I didn't want to sell to him, but he threw down the money and took the goods against my will, and I didn't want to fight with him."

In such a case the seller breaks a full negative commandment: "Do not go as a talebearer among your people." It is true that the second buyer did wrong when he pushed the seller to sell the goods kept for the first buyer. Still, once he sold them to him, it is a complete sale, and no good will come from telling the first buyer the name of the one who pushed. It will only put hatred in his heart against him.

And it is even worse, since it often happens that the second buyer did not push him so much, and the seller sold to him for his own benefit. So that the first buyer won't be upset with him, he takes the blame off himself and puts it on his fellow. This is surely a very great sin, because it is included in motzi shem ra (spreading a false bad name).

One must be very careful in such a case not to reveal the name of the second buyer who bought the goods. Even if the seller takes the blame on himself and says he sold to him by mistake, the first buyer will still feel hatred toward the second buyer for cutting into his livelihood. So the seller should simply answer: "I sold it to someone else by mistake."

Know that everything we wrote in this book about being very careful with the sin of lashon hara is about people who are included in "your fellow" (amitecha). But about those who deny Hashem's Torah — even one letter of it — and who mock the words of Chazal (our Sages), it is a mitzvah to publicize their false views to everyone and to speak against them, so that people will not learn from their bad deeds.