I don't know of any way to lighten the fabric to an exact shade, and am doubtful that it would then fade at the same rate as the original fabric. However, you could explore other options. Use a totally different sashing through some of the quilt - one that is clearly different and not trying to match up. Or if you're early enough in the sewing, use the darker fabric for the vertical sashing and the lighter one for the horizontal. Or use the darker one on the top left hand corner of the quilt (dividing the quilt diagonally). Maybe use one on top, the other on the bottom, and put something different in the middle to separate them. If you're using cornerstones, maybe mix up using the cornerstone fabric with the sashing. You may be starting with a problem, but perhaps you can finish with a "design decision" that will surprise you with how good it looks.