Wordorigins.org

The origin of the almighty dollar is in what is now the Czech Republic. In 1519, a silver mine near the town of Joachimstal (literally “Joachim’s valley,” from the German Tal, meaning valley) began minting a silver coin called, unimaginatively, the Joachimstaler. The coin, which was circulated widely, became better known by its clipped form, the taler. In Dutch and Low German, the initial consonant softened to become daler. English adopted this form, eventually changing its spelling to the modern dollar. From a 1553 letter by R. Morysin and Sir. T. Chamberlayne:

The Duke of Wirtemberg...shall have for his charges 66000 dalers.