Hilchos Rechilus, chapter 9, section 15

Many people stumble in the case described below:
A person brings merchandise to town. Someone agrees to buy it, saying he will bring the money and the seller should keep it for him. Meanwhile, a second buyer persuades the seller to sell it to him. When the first buyer returns the seller says that the second buyer threw down the money and he agreed to sell in order to avoid controversy. By saying this, the seller transgressed, Do not go as a rochel in your nation. Even though the second buyer did a great sin, the sale was valid once the seller accepted his money and there is no benefit in blaming him. All it does is make the first buyer hate the second buyer. This is absolute rechilus as explained above in chapter 1 paragraph 3, and is the same as paragraph 14 in all its details.
It is even worse if the seller did not resist selling to the second buyer and invented the story to deflect blame from himself. This is motzi shem ra, slander and he transgresses all the negative and positive commands explained in the introduction.
The seller should not reveal the second buyers name even if takes the blame on himself and says the second buyer did not put pressure on him and knew nothing of the first sale. For it is very common that the first seller will still hate him for interfering with his livelihood. Rather, the seller should simply say: I mistakenly sold it to someone else.
Know that everything we have written about the great care one must take to avoid lashon hara only applies if the person spoken of counts as amitecha, your fellow. But if he denies God’s Torah, even one letter of it, and mocks the words of the sages, it is a mitzvah to publicize his erroneous attitudes to everyone and to denigrate him so that people do not learn from his evil deeds.