Kant wrote about many things. But let us apply his idea of the Categorical Imperative to business. Kant argued that morality was a priori. This means that be believed rational beings could use reason to determine whether something was morally correct or not. Morality could be determined before experience. In other words, we do not have to "try something out" to see if it is moral or not. We can use reason alone to tell. This ability is what distinguishes humans from other animals, according to Kant. Kant made a distinction between two kinds of imperatives: hypothetical and categorical.
A hypothetical imperative is something you ought to do given your desires. The common example for this is that you should go to the doctor if you want to get well. But one would only follow this imperative when they want to get better. Not all the time. A categorical imperative is one that we must follow all the time, no matter what our wants or desires are. The classic example of a categorical imperative is that one should never use another person for selfish purposes. Such imperatives are universal, they appy to every rational being at all times. I may want to use you to gain access to a better job, but it is immoral nonetheless, according to Kant.
The Categorical Imperative states that we should ask the question "Would that I want to make what I am about to do a universal law? To use the same example above, I would ask myself "Would I want that everyone use each other to gain access to a better job"? Clearly, if everyone did it, it would not be possible to do it, so it is not reasonable and therefore not moral.
So, in business, before you overcharge a client for work that you did not perform, you should ask yourself "Would I want it to be a universal law that all businesses overcharge their clients for work that was not performed? Clearly, the reasonalble answer is NO since otherwise there would be no trust between clients and businesses. Or "Would that I want all advertisments to be misleading and a deviation from the truth? etc.
How is Kant useful to business today? Well, the fact that Kant uses reason, not just desires to determine if an act is morally acceptable or not is something that business can use, and many likely do. Reason and money are not unrelated fellows. But Kant maintains that reason must prevail. Finally, Kant believed that morality implied the recognition of dignity in all other humans. There are likely some businesses out there that value human dignity, and others that do not.