The Premier League and FA fined Sunderland in December for fielding striker Ji Dong-won in games for which he was ineligible, it was revealed this week, but the Black Cats are confident they will not face further punishment.
The club considers the matter closed, Press Association reported, citing sources.
The Black Cats are confident they fulfilled their responsibilities when the administrative error came to light, and insist there is no ongoing investigation amid speculation that the relegation-threatened club could yet be docked points.
Club secretary Liz Coley left her job in December to take up a new position, not as a result of the problem with Ji's clearance, according to reports.
Ji, a 22-year-old South Korea international, was registered with the Premier League and was included in the 25-man squad they released in August, made four appearances in the competition, as a substitute against Fulham, Southampton and Manchester United and from the start at Crystal Palace before the mistake was discovered.
Only one of those games, a 1-1 draw at Southampton on Aug. 24, saw then-manager Paolo Di Canio's side rewarded with a point.
However, Ji also played in the Capital One Cup second round clash with MK Dons, which Sunderland won 4-2 on their way to the final, causing the FA to issue a fine as well, according to the BBC.
Neither the club nor the Premier League was available for comment.
The confusion appears to surround the frontman's return to Wearside after a successful loan spell with German Bundesliga club Augsburg last season.
His spell back in English football, however, proved brief and he returned to Augsburg, this time on a permanent deal, in January this year having made just two more senior appearances under current boss Gus Poyet.
Ji initially joined the Black Cats from Chunnam Dragons for 2 million pounds in June 2011 during Steve Bruce's spell in charge, but started only six senior games for the club, though he did score a memorable winner against Manchester City on New Year's Day 2012.
But while it never quite happened for him on a sustained basis in England, he found German football much more to his liking to the extent that Champions League runners-up Borussia Dortmund were heavily linked with him last summer.
However, Di Canio is understood to have turned down a bid of around 5 million pounds from a German club, only to discover that he could not find a place in his team for Ji, who ultimately survived at the club longer than the Italian.
Sunderland currently sit in 19th place in the table, four points adrift of safety with just eight games to play, following Monday night's 2-1 home defeat by West Ham.