Albert Barnes on 2 Corinthians 12:17

Did I make a gain, etc. In refuting this slander, Paul appeals boldly to the facts, and to what they knew. "Name the man, says he, who has thus defrauded you under my instructions. If the charge is well-founded, let him be specified, and let the mode in which it was done be distinctly stated." The phrase "make a gain," (from πλεονεκτεω,) means, properly, to have an advantage; then to take advantage, to seek unlawful gain. Here Paul asks whether he had defrauded them by means of any one whom he had sent to them.