Cst form on price-difference of earlier years

Hi all,

I have one strange query on C-Form.  One of my client received request to issue C-Form by seller, relating to old years price difference invoice (which may or may not include CST).  Now the questions are:

Is additional C Form required to be issued in case of price variation (Isn't the original C Form sufficient).  Please provide any case law reference, if any

Secondly, if the seller shows CST separately, then can the buyer issue C Form in current year, but relating to old years

Any case-law reference will be highly obliging..

Regards