2 Corinthians 12:17 - Albert Barnes' Notes on the Bible

Did I make a gain ... - In refuting this slander, Paul appeals boldly to the facts, and to what they knew. “Same 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 πλεονεκτέω pleonekteō), 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 anyone whom he had sent to them.