As this article correctly claims, judge Juan Merchan, who is overseeing former President Trump's hush money case did in fact donate $15 to Joe Biden's presidential campaign. He also, according to this article from NBC, donated an additional $20 to two other progressive political action groups during the 2020 cycle. All figures have been verified by the Federal Elections Comission.
Calling $35 an "astonishing" amount of money is a blatant over-exaggeration. Judge Merchan makes over $210,000 a year as a New York state judge; $35 is a tiny, tiny portion of that income. The former President asserting that Merchan is a "Trump hater" is most likely political conjecture meant to fire up his donation and voting base. We can't know for certain exactly why Merchan donated to Biden's campaign, but it does lend some questions as to whether or not Merchan has a conflict of interest in presiding over this case.
Despite nearly 30,000 judges making political donations every cycle, according to that previous NBC news article, Merchan is obviously in a unique situation in that no former president, especially one actually in an election, has been indicted for a crime. However, in this CNN story, Stephen Gillers a New York University legal ethics professor said the trivial amount of donation money would come nowhere close towards a possible recusal of Judge Merchan from this case.