how light is the back of the fabric you already have? it might show the stitches as well as muslin would.
take the fabric you want to use. have a few dollars in your pocket, though, in case there is a "real" need for muslin. if it's a "must", you can pick up what you need at the shop.
or use your uglies and use the dollars to pick up a sale cut after class.
win-win :wink: