The Torah writes (25:14), עמיתו את איש תונו לא, which means it is forbidden to cheat in business. For example, you can't sell something for a higher than normal price or sell something that's broken while telling the consumer that the item is in good working condition. The pasuk concludes (25:17), 'ה אני כי מאלקיך ויראת עמיתו את איש תונו ולא אלקיכם, "...fear Hashem, because I am Hashem your G-d."
Chazal (Avos 2:7) say, דאגה מרבה נכסים מרבה, "When one increases his assets, his worries increase." The Ksav Sofer zt'l explains that this refers to a person who tries very hard to earn more money than that was decreed for him on Rosh Hashanah. He takes on night jobs, works hard, etc. Even if he succeeds and earns extra money, it will be taken away. The Mishnah teaches, נכסים מרבה, if he works very hard to earn more money, דאגה מרבה, he will end up with many worries. He might need to go to the doctor, or encounter other unexpected expenses, so he won't end up with more money than is coming to him.
Why is fearing Hashem mentioned by this mitzvah? The pasuk also says, אלקיכם ה"הוי אני כי, and ה"הוי is Hashem's name for compassion. Why is Hashem's name of compassion mentioned here?
The Maharal Tzinz zt'l (Melo HaOmer) explains that when Reuven cheats Shimon and takes Shimon's money wrongfully (against the rules of the Torah), it is as though Reuven is forcing Hashem to do two things: (1) Hashem has to take away from Reuven the money he earned improperly. (2) Hashem will also need to reimburse Shimon's loss.
It is determined on Rosh Hashanah how much money one will earn and how much money one will lose. That means Reuven has more money than was destined for him, and Shimon has a loss that wasn't destined on Rosh Hashanah.
The Torah writes, מאלקיך ויראת, be afraid of Hashem. You must fear Hashem and refrain from cheating your fellow man because you don't know by which means the excess money will be taken away. Perhaps through unexpected medical expenses or government taxes (see Bava Basra 10). Fear Hashem and the trouble you might have to endure in getting the money back to its rightful owner.
Someone once stole one hundred dollars. The next day, he slipped and broke his foot, and the doctor charged him a hundred dollars. The patient thought to himself, "It's a good thing I stole a hundred dollars yesterday. Now I can pay this bill easily." He doesn't know that if he hadn’t stolen the hundred dollars, he wouldn't have fallen and broken his foot. He broke his foot so as not to exceed the quota that was set for him on Rosh Hashanah.
The Torah adds, אלקיכם ה"הוי אני. This means Hashem's attribute of ה"הוי, compassion, will also be employed because Hashem will compassionately return to Shimon the money that he lost.
The Maharal Tzinz adds: Let's say you know that Reuven stole money or did other corrupt deeds to attain the money of others. And you know that Hashem will certainly take all that money away from him, as it is money that wasn't destined for Reuven on Rosh Hashanah. So, you figure you might as well quicken the process of lessening Reuven's money, and you cheat him in business (ממון אונאת). To counter that logic, the Torah writes, אני אלקיכם 'ה, Hashem says: "Leave these matters up to me. It isn't your responsibility to help remove someone’s extra money. Let Hashem take care of that."
