The following appears in it:
The Jewish Encyclopedia informs us that this Dutch Jew named Schwartzau was actually a Suasso, namely Antonio Lopez Suasso (link):
When William III. undertook his expedition to England in 1688, Suasso advanced him 2,000,000 gulden without interest and did not even ask for a receipt, merely saying: "If you are successful you may repay me; if you are not successful, I will be the loser." Frederick II. of Prussia commemorates this instance of self-sacrifice as the act "of a Jew named Schwartzau.""Schwartzau" was evidently either a sort of Dutch version of his name, or Frederick himself Germanized it, the same way I Anglicized "Friedrich."
So that's the story: William of Orange borrowed 2 million something-or-other from a Jew named Schwartzau, who told him that he could pay it back if he succeeds in his military campaign against England. If not, he is prepared to lose it.
In 1800 an Encylopaedia of Anecdotes was published in Dublin, and here is how it tells it (under the heading Jewish Liberality):
Now it's almost Jew joke: "If you are fortunate I know you will pay me, if you are not, the loss of my money will be the least of my afflictions."