Dear DuoLingo, may I suggest that you check the answer "700 yuan" carefully and don't allow a single mistake in the number. I was told that I was "Almost Correct" when I wrote "900 yuan" - oops! That could be a costly mistake!!!! :)
It is not uncommon for a lot of characters to share a pronunciation so the only way is to see the characters themselves. Sometimes a character will have more than one pronunciation but you can tell from the context which pronunciation to use.
Chinese yuan and United States dollars are two entirely different units. That's like saying 18 kilometers is the same as 18 miles. Besides, the Chinese word for US dollar is "Meiyuan," so 700 dollars would be 七百美元 (Qibai meiyuan.)