Where a dealer is making any purchase or sales, including an interstate sale or sale in the course of export, through an agent or through one of its branches or units in respect of which separate registration has been granted in accordance with the provisions of sub-section (3) of section 20, and the input tax credit, rebate or refund is to be claimed by the agent, the principal shall issue a declaration in form No. 25D to the agent and the agent , in turn, shall issue a certificate in Form No. 25E to the principal.