I think you should just wait until you can return the backpack. Since you are planning on returning it anyway, you shouldn't try to use it for something else.
Did you take your dog's girth and neck measurements? This will be more important for sizing the pack than the dog's weight. The company who manufactured the pack should have sizing information on their website so that you can tell whether your dog is a medium, large, x-tra large etc. Each company is different. Here is a video about sizing for a pack and how to fit it properly once you have one: Dog Pack: Sizing and Fitting - YouTube
While you are waiting, why not measure you dog and research ways you can use the backpack once you get the right sized one?