Have you tried using a calculated attribute? You could add one to to your domain model and use a microflow to add the dollar sign in front of your decimal attribute. You would only use this attribute for display purposes and the decimal attribute for all other calculations/logic.
Your calculated attribute microflow would just need this in your end event. It should be simple enough to not effect performance but just be conscious that calculated attributes can be a performance hit when running complex logic/showing a big data set.
'$' + $Entity/DecimalAttribute
I just confirmed in a sample project that it the export to excel shows the dollar sign.
Just add a new string attribute to your domain model and set the value to be calculated instead of stored.
Here is some documentation on calculated attributes.
Hope this helps!
As far as I know there are two solutions for this problem.