The prohibition of defrauding others is mentioned in two places in the Torah. In one place, it is referring to monetary fraud [ona’at mamon], that is to say, unfair business conduct that causes financial loss to another. In the other place, the Torah is referring to verbal abuse [ona’at devarim], i.e., saying things that cause suffering to others or that deceive them.

A person must be honest and frank in his business dealings with others and must not conceal important details from them. For example, a salesperson should disclose any defects of a product to a potential buyer. Another example is that one must fulfill the terms of a contract, including delivering an object at the agreed-upon time.

When two people agree verbally to carry out a certain transaction, and then one of them retracts his consent before it is completed, the other party does not have a legal claim against him. Nevertheless, the halakha states regarding one who retracts such verbal agreement that “the Sages are displeased with him.” In situations like this, it is best to specify ahead of time that a verbal agreement does not go into force until the transaction is completed.

The Torah states: “You shall not place an obstruction before the blind” (Leviticus 19:14). The Sages explain this to mean that just as a blind person cannot see the path in front of him, so too, one is prohibited from misleading a person in areas where he has no understanding or knowledge. Accordingly, one who gives erroneous or deceptive advice to another has violated this prohibition.