By far the largest congregation of citizens at a base-ball match were assembled at the park of the St. Louis Browns today (June 7, 1875). The defeat of the Bostons on Saturday, the first the boys of the Hub had received this season, had whetted the almost blunted purposes of the St. Louis citizens and, as the games stood 1 to 1, there was, of course, considerable anxiety manifested to see the boys who had dared to defeat the Reds of Boston. The day was beautiful for ball work, and gave a fair and square chance to both sides to show where the skill and ability lay, and every one is wishing to acknowledge the score in favor of the Bostons. There were several rumors afloat this morning that Pearce, the short-stop of the St. Louis nine, had received money from a prominent merchant on Main street to keep himself out of the way of balls, but there was certainly no evidence of such a transaction in the playing of Pearce, who has only one error charged to him. Bradley, who was carried off on the shoulders of six men after the game on Saturday, on account of the victory of the Browns, fell from his usual standard of pitching, being indisposed from an attack of vertigo. The score stands 15 to 2 in favor of the Eastern club.
-Chicago Daily Tribune, June 8, 1875
So this is most likely Ginsburg's source and it is, at the moment, all I can find regarding accusations against Pearce. Not much meat on the bone. There was a rumor floating around St. Louis the day of the game that Pearce was being paid off and then the Brown Stockings got spanked by the Bostons. Well, a lot of teams got spanked by Boston in 1875 so there's nothing unusual about that.
However, if I'm a St. Louis gambler and bet big money on Boston to win the series, I might be inclined to seek some kind of deal after St. Louis shocked Boston the day before and handed them their first loss. The Browns were a good club in 1875 and actually beat Boston (who only lost eight games overall) twice that year. So I guess it's plausible that the game could have been fixed.
And what's up with Bradley? Vertigo? Really? Did Bradley have a history of vertigo? This is a bit odd. It was insinuated that Bradley was involved in a possible fix against the Mutuals in 1876 and here he is in 1875, in another disputed game, having a horrible game and a rather strange excuse for his performance.
The most likely explanation is that the Brown Stockings got beat by a much better club, Bradley had a poor game due to illness and the Pearce rumor was just a rumor. But the damning thing here is that the rumor was making the rounds before rather than after the game.