Betfred founder Fred Done paid out over half a million pounds to United backers in early April when United went five points clear at the top of the table.
However, City are now in pole position to win the league with Betfred themselves only offering odds of 1/10 that they will lift the trophy on Sunday.
City just need to beat Queen's Park Rangers on the final day of the season to secure the title.
In 1998, he lost £500,000 when he became the first bookmaker to ever pay out early on a title race when United were 12 points clear of rivals Arsenal, only to see the Gunners roar back to clinch the league.