I can understand why they won't take the fabric back, but if the machine has been used, it becomes a "used machine" instead of a "new" machine. They can't resell the used machine for the same selling price as a new one. Once I went through two machines until the I got the third machine that I kept. Yes, each time I returned a machine, I did not get the full price back. But, now I have my favorite machine that will probably outlive me.
This was an expensive lesson for me. I learned that one should always sew on a machine at the shop before buying it!