Dentsu Inc. will pay an estimated 230 million yen ($2.3 million) back to customers, including Toyota Motor Corp., that it overcharged for Internet advertisements in a case likely to stoke concern that digital media transactions have lacked transparency.

Japan’s largest ad agency investigated transactions going back as far as November 2012 and uncovered 633 suspicious cases affecting 111 advertisers, including 14 examples where fees were charged and no placement had been made, Dentsu said in a statement. The company apologized and said business results would probably not be materially affected.

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