During the middle ages the Catholic Church forbade usury. Without the ability to lend with interest there would be no trade. That was where the Jew came in and solved the problem for Christian Europe. Both the Church and the nobleman needed an intermediary. Since the Jew was going straight to Hell anyway, according to Church doctrine, why not use him while he was on this Earth.
Consequently, the nobleman lent money to the Jew at 25% interest and the Jew in turn lent it to the merchants or common people at 33-40% interests, which were normal rates in the Middle Ages.
It was a catch-22, however. The non-Jews forced the Jews into being the middlemen in usurious contracts, but since it was against the law the Jews could always be prosecuted for doing so. Inevitably, there would be kings who spent more than they could get and who were always borrowing. They also needed Jews to borrow from. When the time came to call a moratorium on one’s debts, and one did not have the money, an easy solution was to kick the Jews out or kill them.
That happened with regularity. Spain was the place where this pattern first took form.
That is how the Jews became known for their ability to handle money.