I'm not sure if i follow you correctly..
To change the format of sales.. Right click on sales -> Default Properties -> Number Format
Hope it helps!
Can you clarify your question a bit? What exactly are you trying to accomplish?
You want to display a number with decimals, then convert ONLY the decimal part into a String? That is my understanding based on what you've written in your post. Why are you trying to do this? What is the end goal?
Please help us help you by providing more context and describing your goal in as much detail as possible.
Ideally, post a sample packaged workbook with some sample data that demonstrates the issue you are having, along with a mock-up of what you want as a final view in Tableau.