He deals with the customers in a very 'professional' way. = He is very 'professional' in dealing with the customers.
a golf/tennis 'professional' [=a skillful golf/tennis player whose job is to teach other people how to play]
I was impressed by the calm and 'professional' way she handled the crisis.
(..) How's that? Don't you ever do that again or I'll break your head. You got that? I don't work like that. It's not professional. There is rules. And stop saying okay all the time! Good. You expect to use that (..)1994 Léon: The Professional