Use your rotary cutter to cut the binding off. You won't lose much in either width or length. Fuse a patch on the torn spot to stabilize it and then put a new back on the quilt and quilt it lightly to hold the backing to the rest of the quilt and to hold the fused patch in place. That will save the quilt and will add to the quilting design on the front. You will think it is a planned design element.
I'm sorry this happened to you, but you can make a lemonade out of this lemon. froggyintexas