There is a solution, but it requires you to trigger the gold gain. Instead of setting any bounty values on the units, you'll have to make a trigger that catches units dying. Whenever it runs, it gives gold to the killer of the unit and displays the gold gain as a floating text that is non-permanent and has a velocity upwards (90 degrees). Make sure to get a gold color by messing with the color values that is present in floating texts, and you can also set a small text size for it.