By allowing us to choose our passwords and not be dictated to by you on what we want to use.
Yahoo are not very bright, by refusing us to use our own countries financial sign means hacking is much easier as we have to use the Dollar sign. Give us the end user the right to choose our own currency symbol and stop saying its not safe to use when in fact its much safer than using a dollar sign as its a common sign, so is more unsafe than say the British pound. Wake up and fix it as we can find ways of escalating this to higher powers in Yahoo and seriously contemplating doing it.

Hi there! Sorry to hear about the issue. When creating a strong password, you are not forced to specifically use the dollar sign, but any special character, such as the ones from other currencies. Sorry for the frustration.
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James Instone commented
Also stop cancelling our passwords/ We the end user have a right to choose our own passwords even without a number if we choose. Stop dictating to us