Musar zu Schemuel II 23:21

לא תעשוק . refers to the acquisition of something that is not ours by subterfuge instead of outright violence. Whereas in Halachah one is culpable for something which has been acquired by outright robbery only if it is worth at least as much as a certain coin called פרוטה, cheating by using unfair weights is punishable even if the amount by which one has cheated is totally insignificant, much less than the value of a פרוטה. The decree to bring on the deluge was sealed on account of the sin of robbery. We have a saying (Vayikra Rabbah 33,3) that סאה מלאה עונות וגזל בתוכה מקטרגת, "in a סאה, a given amount of sins (about 800 cubic inches), if it contains even a minute amount of the sin of robbery this quantity turns into an accuser against us." I have observed pious people who, when they purchase something for a holy purpose, always pay the asking price and never haggle so as not to be remotely guilty of transgressing these commandments of לא תעשוק את רעך ולא תגזול. They are so similar that when one transgresses one there is a good chance that one becomes guilty of transgressing both, becoming guilty on two counts. Technically speaking, לא תעשוק means to withhold by force something someone else is entitled to.