The Congress on Thursday said the CAG had no mandate to audit the accounts of private companies and sought to debunk the argument that the government should have opted for the auction route.
The party also asked the CAG to respond to the debate triggered by its findings about its methodology and working style.
Quoting a Supreme Court order in April this year, Congress spokesman Manish Tewari said the CAG can audit any private company only if asked by the government to do so.
"From where did the CAG get the authority to audit the private companies, which got coal, and arrived at an estimate? There are a number of unanswered questions with regard to not only methodology but whether CAG could have conducted the audit."
However, opposition parties questioned the government's stand saying that any deal that involves public exchequer automatically comes under the Prevention of Corruption Act.
However, he ruled out any possibility of impeachment of the CAG.
"You have to make a distinction between the CAG and its report. We are disputing the report of the CAG and the methodology he has adopted. There are repeated averments that you are attacking a constitutional authority. It is not inappropriate to raise legitimate questions on what the CAG has done."