The right operator for unit conversions is "in." "to" is used for ranges, so it's doing 100 euros _goes to_ 1 us dollar and giving you back the answer in dollars.
But it seems the parser is breaking when trying to do the inverse, anyway: "100 usd in eur" seems to do "e*ur" and gives "Values must be converted to units."
But it seems the parser is breaking when trying to do the inverse, anyway: "100 usd in eur" seems to do "e*ur" and gives "Values must be converted to units."