if he was an extra db on passing downs, that is what we call a nickelback, which can be classified as corner. Only recently have I seen nickelbacks being called safeties, surprise, all of the ones I have seen are White.
Now this is usually the case, now if a safety is used as a nickelback, he probably has enough talent to play corner. When its a White guy playing the nickel, 90 percent of the time he is listed as a safety because of his skin color. Contrary to popular belief, its not easier to play the nickel. Guys like Revis struggle to play on the inside.
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In American football, a nickelback is a cornerback who serves as the fifth (in addition to the typical four) defensive back on the defense."