I thought about this.
The only way I found is by creating a mixin :
@mixin setBgColorAndHover($baseColor)
background-color: $baseColor
&:hover
background-color: darken($baseColor, 5%)
And then :
.button
+setBgColorAndHover($green) // as $green is a color variable I use.
Not the best, but that will do the job 🙂