Hilchos Rechilus, chapter 6, section 3

If he had a loss in his business and does not know why, it is forbidden to suspect a fellow Jew. About this it says: "In righteousness shall you judge your fellow."

Even if he heard that a certain person caused the loss, it is forbidden to believe it. He may only be careful.

Even if people accused that person to his face of the wrong he did, and he stayed quiet, this is no proof that it is true. Perhaps he saw that the listeners were so sure he did it, that he preferred to be one of "those who are insulted but do not insult back".