Musar על שבת 237:2

When the Torah speaks about not stealing, (19,11) (compare negative commandment 155 according to the Semag,) this does not refer to the theft of something tangible, but to what our sages called גנבת דעת, false pretenses, trickery. Such false pretenses are prohibited even in one's dealings with a pagan. This is discussed in Tur Choshen Mishpat section 228. The basic prohibition of גנבת דעת is expressed in the words: לא תשקרו, "do not lie." A person is not permitted to deny his guilt by claiming he had merely made a mistake. This is why the Torah says: לא תכחשו, "do not deny." The Talmud in Shabbat 119a tells of a butcher who had become extremely wealthy and who attributed this fact to his having saved the very best part of his animals for the Sabbath [but for his own and his family's consumption. Ed.]. This does not contradict the statement that the best part of one's goods should be presented to G–d, or, failing this, to charitable causes. The Talmud merely mentions the incident to demonstrate how great is the merit of honoring the Sabbath and that he who honors the Sabbath for its own sake will be rewarded.