Which player has scored the most goals in a single FA cup match and still been on the losing side?

Denis Law, when playing for Huddersfield, scored six goals in a game which came to a premature end because of bad weather. When the replay came, his team lost.

Wilfred (Billy) Minter - seven for St.Alban's City against Dulwich Hamlet in a qualifying round on 22nd November 1922. St.Alban's lost 8-7.

On 22 November 1922, Wilf Minter scored seven goals in an FA Cup match and still ended up on the losing side. The match was an FA Cup Fourth Qualifying Round replay, pairing Dulwich Hamlet from the Isthmian League against Minter's side, reigning Athenian League champions St. Albans. Their first meeting ended in a 1-1 draw at St. Albans, with the hosts nicking a late equalizer.

Played before a crowd of 4,060 at Dulwich's Champion Hill, the replay featured emergency replacements in both goals, which undoubtedly contributed to the scoreline. After the first 30 minutes, Minter had his first hat-trick and St. Albans were leading 3-1. Dulwich rallied to go ahead 5-3 in the 60th minute, but Minter notched another hat-trick in the next 10 minutes to push St. Albans ahead, 6-5. With 5 minutes remaining, Dulwich scored again to send the match to extra time. Dulwich struck first in extra time, but Minter again rose to the occasion, scoring his seventh to equalize five minutes from time. The teams appeared to be headed for another replay when a Dulwich player named Davis scored his fourth of the day to win the match 8-7.

http://tdifh.blogspot.com.au/2010/11/22-november-1922-minters-unlucky-number.html

Competition proper - Denis Law 6 which did not count for Manchester City against Luton Town in an abandoned 4th Round tie on 28 January 1961. Luton won the real tie 3-1.

Denis Law actually scored seven goals in the same round of the cup. He scored six in the abandoned game when Manchester City were beating Luton 6-2. In the re-arranged game Luton beat City 3-1 with Denis Law scoring for City. Unfortunately, the goals scored in the abandoned match were not part of the official records.

https://www.mancity.com/news/club-news/club-news/2019/city-dna/city-dna-36-denis-law-scores-six-goals

Four players have scored two goals in an FA Cup final and been on the losing side.

1981 - Tommy Hutchinson of Manchester City scored an own goal for Tottenham and then equalised for City. City lost the replay.
1946 - Bert Turner (Charlton Athletic), scored an own goal in the 85th minute for Derby and then he equalised in the 86th minute. Derby won after extra time 4-1.
1989 - Stuart McCall came on as sub for Everton and scored twice, however Liverpool won 3-2 after extra time.
1990 - Ian Wright scored twice for Crystal Palace against Man Utd. the match ended 3-3 aet, with Man Utd winning the replay.