Install and launch [Karabiner][1]. In the menubar open the menulet Karabiner -> Preferences. Then hit the tab "Misc & Uninstall" and "Open private.xml"

Paste the following lines into the opened xml file:

	<?xml version="1.0"?>
	<root>
		<item>
		  <name>Swap Minus and Alt+Shift+Minus</name>
		  <identifier>remap.minus_and_optionlshiftlminus</identifier>
		  <autogen>
			__KeyToKey__ 
			KeyCode::MINUS, ModifierFlag::NONE,
			KeyCode::MINUS, ModifierFlag::SHIFT_L | ModifierFlag::OPTION_L
		  </autogen>
		  <autogen>
			__KeyToKey__
			KeyCode::MINUS, ModifierFlag::SHIFT_L | ModifierFlag::OPTION_L,
			KeyCode::MINUS,
		  </autogen>
		</item>
	</root>

and save the file.

Change back to the tab "Change Key" and enable "Swap Minus and Alt+Shift+Minus". It should be the first entry listed.

If you are left-handed you may replace _L by _R in the xml file.


  [1]: https://pqrs.org/osx/karabiner/