I have had good luck using wrinkle release on fold lines to remove them. If faded then I expect the seller to make it good.

I agree with contacting the seller for sure. Is the fold line where maybe it was exposed to the sun? Sorry to hear this-as you say-it wasn't worth getting the wide backing if you have to cut and sew back up!!

Your question doesn't quite clarify whether this is a fold or a line of fading; thus the mixed responses. I hope that seller makes good on this for you. It isn't going to get better with time if it's faded. If they have a rule, "no returns after washing", at least you have provided a heads up for the rest of us to pay attention to this possibility.

I thought of another idea to deal with it: If you have decorative stitches on your machine, perhaps the line would provide an interesting background to highlight a special stitch and turn it into a feature rather than a flaw. That way you wouldn't have to make a seam. I'd do this before sandwiching - not so it would show on the front of the quilt. It would work along a fold or a faded line, either of which would provide an easy guide for your sewing.

I , too, hope the seller sends you a new piece to work with so your backing can be done the simplest way. Whether a permanent crease or a fade line, you shouldn't have to accept it for full price unless it was advertised or pictured with that flaw.

I like the suggestion of sewing over with a decorative stitch before sandwiching. I also like the idea of folding it instead of cutting, and then just stitch a seam and cut off the excess. Another remedy (requiring more work but visual interest) would be to cut out a strip with the flaw and sew in a coordinating fabric. I often do this because I like pieced backs or my backs have been too small to start with. I haven't had to do it as a result of a poor piece of fabric.

Sounds like a fade line...been there done that...contact seller and return.