Hilchos Lashon Hara chapter 5, section 1

Just like it is forbidden to embarrass a person regarding items that are between man and God so too is it forbidden to embarrass a person regarding items between man and fellow man. Even if the story is true and even if you saw one person ask friend to lend him money or to do a favor any type of favor and the man is not agreeable to do the favor- if you go and you and malign the person who did not agree to do the favor for the friend this is Lashon Hara in its truest form and all this is even if you saw this yourself and you knew that it was possible for the man to do the favor for the friend but the man has bad character traits and was not agreeable to help. If the victim tells over the story he violates לא תטור and if his intention of repeating the story is revenge he’s also violating לא תקום