How to select keyboard input file for Add Input Source
The underlying confusion here is that an existing keyboard layout file may not be modified in place.  You need to delete it or rename it then add it back.

The Input Sources program actually reads the xml's of the files uner

~/Library/Keyboard\ Layouts/


It will not allow you to re-select a layout with the same name of one it already knows about.  Unfortunately that prevents updating an existing one using the existing name. 

You have to update the layout name in the xml:

&lt;keyboard group="0" id="5000" name="Dollar Layout" maxout="1"&gt;
    &lt;layouts&gt;