Demonstrating how you can redefine placeholder selectors in sass.

gistfile1.scss

// Define Button
%button { background: red; }

// File #1
button, a.button { @extend %button; } 

// File #2
input[type=submit], #some-classs { @extend %button; }

// File #3
input[type=another], #some-other-classs { @extend %button;  }

// another file with block that needs special buttons, but you don't want 
#custom-block {  
  %button { 
    background: green;
    // redefining %button here will reprint everything with %button extends with #foo class in front
  }
}

// output

button, a.button, input[type=submit], #some-classs , input[type=another], #some-other-classs { 
 background: red;
}

#custom-block button, #custom-block a.button, #custom-block input[type=submit], #custom-block #some-classs, #custom-block input[type=another], #custom-block #some-other-classs { background: green; }