There probably are tools to do the reverse compile. I am not familiar with any personally. I would suggest you look for any .MEM files in the application directory. This is a file where memory variables and their values can be saved in dBase. If I were writing such an application, I would save a parameter like sales tax rate (which might change over long periods of time) in a variable saved to a file. If you don't have the dBase application to modify the variable value, I have had good luck simply changing the variable value by opening the file in Notepad.
Maybe you'll get lucky and that's how the sales tax value was handled, as opposed to being hardcoded in the .PRG.