There are so many "ifs" in this situation:
if I really like the fabric
if I think I could use it for other projects
if it's not too expansive
then I would probably keep it.
On the other hand, if it's project specific, expensive, and if the staff is accommadating, I would probably ask if they would cut off the 2 feet I need and keep the rest. I believe the people at my favorite LQS would because they could always us the left over for FQs, charms, etc. I'm not so sure about someplace like JoAnn's or other chain stores.