How relevant to business and management is a Kantian approach to ethics?
To quantify relevance is difficult, but I can to show you why Kant's and business may go hand and hand.
Businesses are generally set up to make money. Anything that is not practical or does not show a ROI (return on investment) is considered something to be avoided.
Just as business are clearly set up to show a profit, Kant's ethics are also rule based and founded on Reason. Most business are founded on logical principles and are also rule based.
Kant tried to come up with an ethical rule that was not relativistic; one that would be the same for everyone, everywhere.
This is what is rule says (simply put): Before you do anything, ask yourself if you would want everyone to do it. So, if you are thinking of stealing money from your job, ask "Would I want everyone to steal money from their job?". If you can answer 'yes', and such a rule would not lead to a contradiction, then it is morally permissable according to Kant. He called this idea the Categorial Imperative.