general.sls

{% set source = 'default' %}

{% if grains['a'] == foo %}
  {% set source = 'foo' %}
{% endif %}


/a/b/c.conf:
   file.managed:
     - source: salt://{{ source }}
     - mode: 0700
     - user: root

specific.sls

include:
  - general

{% if grains['b'] == 'bar' and grains['a'] == foo %}
{% set source = 'foobar' %}
extend:
   /a/b/c.conf:
     file:
       - source: salt://{{ source }}

{% endif %}