Freezing Butternut Squash
Another vote for roast, then freeze. Freezing will make the water in the squash expand and burst cell walls, creating mush, so you might want to puree, or even make the whole soup, then freeze. I have several frozen containers of butternut soup right now--it thaws and reheats perfectly.
When did you buy it? In a cool, dry place, hard-skinned squashes last for months. I recently made curried squash and apple soup from a butternut bought at a farmstand 2 months earlier. It seems to me that the flavor actually develops with storage. Last year I made the soup right away and thought the squash had less taste.
Sounds like your recipe requires the squash be roasted. If so, do that first, then freeze. If not, cut it up and freeze it raw. It's easier to peel if you nuke the whole squash on medium heat for a few minutes - this won't be enough to cook the flesh but will soften the peel enough to make it easier and safer to peel. Or, make the soup now and freeze - pureed squash soups freeze very well.