The C should prove the genuineness of the transaction and present the evidence for this like invoice , challan , GRN , Bank payment detail etc . If C has acted bonafide , and prove the related evidence , then he may be out of the tax net .
Any such demand can be challenged before the appellate authority as per MVAT Act or in suitable case file a writ and get a suitable order restraining the dept not to take any coercive action .