Musar su Salmi 18:27

And if it occurs that he anticipates some loss to himself as a result of this advice, then if he is able to admonish the advisee directly, he should do so. Otherwise, he should withdraw from the matter and not give any advice. In any case, he must not give an advice whose purpose is other than the benefit of the advisee, unless the intent of the advisee is evil, in which case it is certainly a mitzva to deceive him. And scripture already said: "but with a crooked one, You deal crookedly" (Tehilim 18:27), and the story of Chushai the Archite demonstrates it.

Laban was possessed of עין רע, he was ill-willed, begrudging, he was haughty and greedy. His whole concern in approving the match was to secure personal gain. The Torah is careful to document how Laban was motivated by money when we are told: "As soon as Laban saw the nosering and bracelets on the hands of his sister etc.,… he ran towards the man" (24,30). Laban was a swindler to boot. Realising that Eliezer had come to propose a שידוך, a match, he whispered to Eliezer that in consideration of an appropriate amount of money he would side with him and approve the match. Should Eliezer refuse, he, Laban would oppose the match; he was the most influential member of the household. When the Torah reports that "Laban had heard the words of Rebeccah his sister," this is our clue that he knew that a match was going to be discussed. Eliezer had no option but to promise Laban what he had asked. The reason the Torah does not report all this is because it simply never came to this. Eliezer did not keep his promise, as I shall explain. In his dealings with Laban Eliezer applied the principle of עם עקש תתפתל, "with the perverse you have to be wily" (Psalms 18,27). He did this by making a promise he would not keep. Laban was misled, for he believed that Eliezer would keep his part of the bargain.

When they said: "The brother of the master was a swindler," they deliberately referred to him as "brother" instead of saying "Jacob the swindler." Their intent was to make Jacob's trickery appear to have been greater than it was. They referred, for instance, to the time Jacob introduced himself to Rachel as the brother of her father, (the swindler), something which Rashi justifies as Jacob telling Rachel not to worry, that he, Jacob, would be able to cope with devious Laban (Genesis 29,12). Whereas the Romans could see Jacob's trickery as justified in the case of Laban, since Laban had been the first one to use trickery, this could not be said in the case of their Master Esau, who had never tried to cheat Jacob. Jacob cheated Esau out of his birthright as well as out of his blessing; because of this they emphasized that Jacob was both a swindler and a forger.