Is there any way to have the "say" command say a negative money amount? I get an error (ERROR: Money '-21.78' has invalid character '-') when I try. I know I could do a basic script to remove the negative sign then say negative before I say the money but this might be hard with the way I have designed my flow chart.

The say step could be updated as you describe, yes. We have added this to our engineering queue, so it will be addressed in a future update.

Unfortunately, I cannot at this time provide an estimated time for delivery. For your purposes, your best bet is to do what you have suggested, and remove the - sign, and use logic to play a prompt indicating a negative amount.

If the prompting can be the same every time, I would actually recommend creating a flowchart for this task.

You could actually do something like this:
Have a flowchart named something like 'Say_Currency'. Anywhere you are currently using a Say step to speak a currency value, replace the step with a Run Flowchart that runs 'Say_Currency'. Pass the value to play into the flowchart in the first parameter. You can test the value with a Compare, see if it is < 0. If it is, use Run Basic Script to strip the - sign and play "negative" then the say. Otherwise, just run the Say to play the value. This allows you to create the logic once and use it anywhere you need it (including other applications).