A Prompt module with a currency variable reads back the currency value the same whether it's a positive or negative number. It doesn't appear to support negative numbers. This limitation isn't mentioned in the documentation so I think this is a bug. I've tested with both a JavaScript native number and string variable.
Luckily this can be worked-around by altering the flow in case of a negative number using the Compare Variable module and manually saying "negative" before speaking the currency value.
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Currency prompt does not support negative amounts
Re: Currency prompt does not support negative amounts
Thanks for helping us find this bug. We are working on a fix and we will let you know when it is deployed.
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