According to the National Enquirer, Affleck returned a $250,000 (£166,670) payment to the Greenbrier resort in White Sulphur Springs, after realising that he had been paid twice for a personal appearance.
Casino owner Jim Justice thanked Affleck, saying: "We mistakenly paid him twice. Fortunately, he called us or we never would have caught it.
"Obviously, we've got a few teething problems in our accounts department that we just need to clean up.
"We're just grateful that Ben Affleck caught the mistake and told us about it. He's a very honourable man."